Etsy.com is another great resource for costuming. All finished products are (supposedly) handmade (although manufactured goods periodically sneak in, contrary to the site's terms and conditions). Why is this good? Because it means when you show up in something bought on Etsy you're unlikely to see twenty other people wearing the same piece. Many sellers are also willing to work on custom pieces for customers.
My suggestions here only scratch the surface. There's a lot of quality steampunk items to be found here. The shops do have a habit of disappearing, however.
Read more: Tips for Online Shopping
CatherinetteRings - I absolutely adore this site. Unique pieces reasonably priced. "Like" them on Facebook and get notifications of various sales such as buy one get one free.
Highwind Steamworkx - Affordable goggles, plus a variety of handmade jewelry including old watch parts. They also show up at steampunk events in the Midwest.
London Particulars - Jewelry made from watch parts, average price about $50.
Lily's Steampunk Emporium - Decorated goggles for $45. At conventions she also has a wide selection of really exquisite broaches, also reasonably priced at $35-$40.
Steampunk Jewlery Designs - What originally caught my attention was their steampunk hair adornments, although they also have affordable pins, rings, cufflinks, and more.
Gypsy Lady Hats - If you're looking for a creative hat, these people have a lot to offer. Prices range from $60 to $200, with most falling within the $85-$150 range.
The Blonde Swan - Primarily a hat store that is pricey but popular. While you can find the odd, basic hat for as low as $30, the majority of their hats are $100-$200.
Lilly's Workshop - Handmade tapestry and faux-leather corset $180-$245. Items are custom made for your measurements.
Steampunk Supply - Offers an affordable array of parts, mostly watch parts and keys, for use in steampunk creations.
Alt. Kilt Steam Shop by TechDragon - I love them for their reasonably priced pouches and bandoliers. They also offer a smattering of other items.