When I was out at Steampunk November one of my friends spotted a woman wearing a very nice lace-trimmed peasant blouse and said "I need a shirt like that!" She asked where the woman got it, and she said "Oh, Hips and Curves!" And then there was a conversation about how great their underwear was.
Luckily, I'm an affiliate for Hips and Curves, which means if you buy from them after following the links from my blog, I get a commission. So I went to find this shirt, and was really surprised how much great steampunk clothing the site has. It's been a while since I really looked at it. Now some of their stuff is going to be costume quality, and I don't necessarily vouch for their corsets (I believe they are of the same quality as Corset Story, and you can read my opinion of those here.)
But they also have some pretty decent pieces to add to your steampunk wardrobe.
This is the item that started this. Hips and Curves calls it a dress, but I'd wear it as a top, personally. Maybe with leggings. But it looks great under a corset. And the fabric is very flowy and gauzy, which is nice for warm weather. Sure, maybe this is more Ren Faire or fantasy than Victorian, but we all know the fashion of steampunk is broadly inclusive.
I have a pair of striped bloomers I made that I love, and have taken to wear frequently when I want to be comfortable and cool but still dress steampunk. These white bloomers are similar and I would recommend them to women who want to forgo long skirts and get a little more comfy in their steampunk.
tiered ruffled petticoat skirt either. They also have a similar skirt in black.
The purple velvet Alice Revolver jacket is really lovely, with a corseted back and frilly tails at the back. They also have a similar look in black brocade with the Bowie Brocade Jacket.
This Cotton Tiered Petticoat is nice for it's simplicity. You could wear it for everyday wear, alone with steampunk clothing, or as a base under other costume pieces.
That's really only a taste. There are a lot of petticoats, accessories, and potentially cool pieces in the costume section. It would be a good place to get some base pieces for a steampunk look, particularly if you have trouble finding things in plus size, or live somewhere that you don't have access to a festival to buy from vendors.