Antiques in San Jose, CA

There are many resources - some basics: https://www.steampunkemporium.com https://www.gentlemansemporium.com Corsets are everywhere - and personally I'd avoid the standard 'steampunk corsets' that are really easy to find right now: http://www.corset-story.com/style/steampunk.html but that may be what you're looking for! (totally fine - I just like to personalize my look a little more.) I really like http://damselcorsets.com/. And then I'd recommend just doing a search on etsy with 'steampunk' as the keyword - it's sort of a good way to start generating ideas and there are some awesome craftspeople there. Good luck!

Renaissance festivals are a good place to go as well.

For actual old garments, antique stores are worth a look. A friend came out of one with an Edwardian dress that actually fits her -- and it only requires a bit of repair. A good less-expensive retailer for some basics is Wild West Mercantile store.

For off the rack corsets, I highly recommend timeless trends and Orchard corsets. They have great ones and you can get basic fabric or brocade or whatever to fit your look you're after.

there are vintage clothing places in your local area most likely.

Here's a guide on how victorian fashion is used in steampunk fashion. There's a lot more info in the skirt and dress section than there is in the corset section (since corsets weren't worn outside of clothing like they are in steampunk fashion): http://www.steampunkfashionguide.com/p/steampunk-fashion-guide.html