I just made a post on my blog about this, with links and prices and such, but I wanted to share the information here, too.
After looking through everything Forever 21, H&M, and a couple of other stores had to offer, these are the most loliable blouses I was able to find. You can visit my full blog post if you want (http://ravensnow.wordpress.com/2014/11/24/offbrand-blouses/), but here are the pictures for you to look at.
(The last one, I think, would work well with more otome or retro-inspired lolita.)
I think that all of these are loliable without needing any extra modifications, except I would only wear them underneath JSKs or high-waisted skirts. These boxy blouses look really weird to me in lolita when they're worn with a regular skirt.
EGL, what do you think about using Forever 21 and H&M blouses in lolita? Are there other stores you like to go to find offbrand blouses? What are your thoughts on offbrand blouses in general? What about nice offbrand cutsews?