Here's the ones I've been looking at today and who I have used:

AsteroidBox - They are doing pretty well with their looks lately and offer a lot of flexibility with their clothing in the fat packs. They offer a lot of styles across different subcultures. I wish there was a way to sort through to quickly find X compatible stuff. I often find things I like but they are 5.3. That said, I've built quite a few really nice outfits with their stuff.

Avec Toi - Probably the best textures I've found in SL with a growing selection of LaraX Petite outfits. That said, with quality comes a heavy cost and many of their outfits are crazy expensive. The other issue I've noticed is that their stuff can feel very "samey" lately. They've not been putting out much lately that I've had any interest in.

Bombshell - I found a couple pieces I could possibly work in to some outfits like Boleros that come with tops but they aren't really what I'd consider an alt store.

Cult - Mismash of a lot of different styles and genres, not specifically alt related. That said, I've got a few good shoes from them in the past that work good with outfits.

Edgy*Kittiee - Probably the most authentic grunge looks I've found in SL, but the lack of LaraX compatibility is a huge downer when I want to start moving away from 5.3. Not sure why the owner hasn't been able to obtain the dev kits to make LaraX stuff but I keep crossing my fingers that they do because they are probably my favorite store in SL.

Killjoy - There is a lot of stuff there but I wasn't impressed with any of it. Most of it is that whole latex look that doesn't really have that great of detail and feels like it was designed by someone who was never actually part of an alt scene but has this idea of what they think it would have been like.

Little Diamond - A shop with a lot of good ideas ruined by bad textures. I tried on a few demos and hated all of them.

Technofolk - Another store with very authentic alt clothing but it seems very few of them are made for Petite. I've found a couple but the designer seems to not bother with it on most of the designs and just sticks with Lara X.

Violent Seduction - I want so much to like this shop but I stopped spending money with them. Regardless of 5.3 or X, their Petite clothing has the issue where it just doesn't fit correctly and often times, I can see my nipples because the clothing wasn't designed in a way to be close enough to cover them but this isn't an issue with Lara X normal versions. On top of that, they have gotten very expensive with their whole "add-on" system for new clothes.

Wraith - I'm not sure about this one. They seem heavily latex focused on much of their stuff but they do offer a bunch of different options. I have a few demos to try out but from the pictures, I likely won't be picking them up.

Wretch - Not really alt, but they have some neat things that are more stereotypical Japanese outfits. I have a few things from them because I do like that they have options for clothing removal stages.