As someone who likes affordable and decently up-to-date fashion, I always default my choices to UNIQLO or GU. For most people, both of these stores are readily available at their local malls - whether you're in a big city, or in a small town. And they have a good variety of sizes, even for those who are not "slim" to Japanese standards.
Unfortunately... Japan is very "one-dimensional" when it comes to sizes. Those who are larger... yes, I am sorry but this list will probably not be so helpful for you, as that's just the reality of clothing in Japan. I wish I could change that. And there are some options, such as Shimamura. However, I just want to make it clear that this list will mostly be for those who aren't blessed with too many curves and awesomeness.
I'll try to include online shops if they are available, but most of these shops will probably be available to some degree in a tourist area, an outlet mall or a decently-sized mall.
This is one of my personal favourite Japanese clothing. I like the pastel colours they have in their collections, which is quite cute for both casual and semi-casual for work. I recommend this shop for those of you who, like me, have issues getting more colours into their wardrobes (black clothes ftw!). I think pastel is a good segue into more colours.
They also have some really nice anime collabs from time to time, and I recommend those of you who like anime to keep an eye on their stocks every once in awhile!
2) INGNI
I like this shop a lot because they're quite affordable, and they have options for work and casual wear. Their clothes are also leaning more towards the cute side, but they sometimes do have sexier styles for ladies looking for date clothes and what not. I enjoy the look of their clothes, and very easy to pair with other neutral colours.
3) Honeys
I've recently really started to like Honeys a lot. Their stores tend to carry a lot of different stocks and different designs for similar items, so I feel like there's always something for everyone. The items are usually quite fashionable, and also work-friendly too.
The problem for me when I'm usually at a physical Honeys store is that I get extremely indecisive about which design I want on the blouse I want to buy, and I spend almost way too long at the store... which I guess is a good shopper's tactic? XD
4) Global Work
Global Work is one of those places where I wouldn't say it's cheap, but if you need something strictly for work, it wouldn't be too hard to pick out a set of work-ready clothing in the shortest amount of time. This shop is geared towards office-style clothes, and they have a decent selection for both men and women. I wouldn't say good as it lacks variety, but like I said... if you hate shopping, and you just need something quick, this place's got your back.
Global Work is part of the subsidiary with LOWRY FARMS and niko and... which are popular semi-casual fashion brands here in Japan. They all kind of sell similar things (at least to me), so in most cases it comes down to price and quality (which... idk, is similar to me as well unless things go on sale).
5) Right On
This store actually is more for men and carries more casual & streetwear. They also carry some western brands as resellers like Lacoste and Levis, but sometimes you can find some seasonal clothing that are both good for work and casual outings.
For me, this store has that western vibe and western fit, but with a more Japanese twist? I like how their clothing are mostly made for comfort, and loose - but also fashionable. Work clothes tend to be more about the tight fit, but if you work at a school like me and you can afford to be a bit more casual with clothes, I definitely recommend checking Right On out.
A couple years ago, I bought these thick black winter pants that look like jeans (they're not). They were extremely fluffy, comfortable and warm on the inside. It tucked well into my boots and weren't tight - which was great. I've since lost these pants (I think I might have discarded them by accident), but I still think about them a lot...
This store carries more "cool" and "hip" styles compared to the ones on the top of the list, though a lot of the clothing varieties aren't all that different from the rest. I particularly like getting neutral-coloured tops and knits from this shop when things go on sale. They have some very comfortable styles that are easy to match.
When their items are not on sale, they can be on the pricier side - but still relatively affordable. I generally like their material and quality too, so even if the price is slightly higher, their products do last a bit longer than getting something similar at GU or UNIQLO.
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To be fair, in Japan... there's a lot of clothing stores that stock on very similar items, but then it really comes down to how much you're willing to pay, the quality and a few difference in details. Like the neckline, range of colours, decorative pieces, belts, etc... However, I think all the places I listed are definitely affordable and slightly more trendy than something from GU or UNIQLO.
In terms of quality, I feel like cheap stores like INGNI definitely lack that. For the other stores, there's a lot of hit and misses. Some have lasted me for at least 2 years and up, while others from the same store just don't seem to last at all.
Definitely go into the stores yourself and have a look (after this pandemic is over, of course - not now!). I hope this helps some of you expand your shopping choices a bit more.