Are you looking for layer or mesh clothes?
If it's mesh, Damien Fate is your friend. Try Deva Moda in the big yellow house in the southeast corner of Klamotto (
https://opensimworld.com/hop/78282) and the Meshanthropy store on the HG Safari sim (
https://opensimworld.com/hop/75662).
If it's either, look around the Astralia ShoppingCity (
https://opensimworld.com/hop/85405) where you can find more Damien Fate among other things. You may also check out all three floors of the Kaufrausch Mega-Store in Santiago (
https://opensimworld.com/hop/78061), but beware: Some of the mesh clothes on the top floor are rigged for Ruth 2.0 and will be too tight for the systembody. Furthermore, take a few hours to check out La Tortuga (
https://opensimworld.com/hop/76639). It's basically Clutterfly, but with loads of other classic (and mesh) clothes added.
If it's classic/layer, I know countless sims where to get standards as well as exclusive material, but almost none of them are on OSW. Some are. I recommend you to look around two OSCC sims, namely the Welcome Center (
https://opensimworld.com/hop/75963) which has a lot of FleepGrid content next to widespread full avatars and Linda Kellie and Beth Ghostraven's shop in the Expo Zone 5 (
https://opensimworld.com/hop/75968). The TanGLe Welcome Center in Diamond Ridge (
https://opensimworld.com/hop/76514) mostly offers FleepGrid and Linda Kellie, too, but in the latter case, also some post-Freebie Mall 2.0 content (see TexLand Shopping Mall,
https://opensimworld.com/hop/77233). Anna Barzane offers a few exclusives, including underwear and swimwear, at Les pieds de sable (
https://opensimworld.com/hop/79417). Furthermore, look around the TPW-Mall (
https://opensimworld.com/hop/82973) and of course Wright Plaza (
https://opensimworld.com/hop/75040).
As for places with no OSW presence: The best source for Linda's stuff is her Avatar Center (for example
hop://hg.osgrid.org:80/archiAvatar/127/123/22 or
hop://hg.osgrid.org:80/Free Mall 3/129/128/22). AFAIK, it was her last sim for avatar accessories and clothes before she went mesh. If something should be missing, check her Freebie Mall 2.0 as mentioned above. Dereos offers enormous amounts of exclusives: Start at the PSSMG Mall (
hop://dereos.org:80/PSSMG Mall; can't taxi you to right in front of the building), pick up skins and mesh footwear in Lys Sandbox (
hop://dereos.org:80/Lys Sandbox/220/130/23) and then head west for D&J Designs with more layer/prim clothes. For more exclusives, take yourself some time to visit Festa 24H (
hop://hg.osgrid.org:80/Festa 24H/) with its many shops. AviWorlds has an entire sim dedicated to classic content (
hop://login.aviworlds.com:8002/AviWorlds Freebie Mall Oldschool Items). And there's a lot to find at the Craft Historical Shop (
hop://craft-world.org:8002/Craft-Store/98/196/51; you may have to use a teleporter to get there, and the women's fashion is on the white helix).
Finally, here comes a special section for hosiery. I think it's worth it because what feels like less than a dozen female avatars ever wear stockings and/or tights, so all you BoM ladies pay good attention, too. When you're at the Craft Historical Shop, go all the way up the helix. Go to the Body Shop at Zoe's Mall (
hop://2.nymphgrid.com:8002/Oceanid/428/575/22) to find Shinobar lingerie and hosiery boxes, one of which even Shinobar doesn't offer anymore. When you're at Wright Plaza, visit oopsee Creations (
hop://hg.osgrid.org:80/Wright Plaza/170/68/21), and then pick up Morgaine Alter's autumn-themed tights (see them here:
https://opensimworld.com/post/81425) from the mixed freebie area (
hop://hg.osgrid.org:80/Wright Plaza/64/207/22); you can also find them at Morgaine's own shop at Festa 24H (
hop://hg.osgrid.org:80/Festa 24H/206/184/21).