My first corset (sounds like a bad fetish toy
) was a $130 basic number from absolutecorsets.com, and while it's a pretty nice corset (shameless plug - go see my Tilted Exhibition thread
) it could fit better. Granted, I lost enough weight that, as ratbastid said, fully constricted it doesn't bind much now, but it never really worked up top. Absolutecorsets does offer custom corsets but you have to submit your own measurements. If you have a custom shop around, go for an in-person fitting. Otherwise, order a size that's 3-6" below her natural waist size.
I have a couple of other corsets that were ebay specials (of the cheesy Frederick's variety, see aforementioned TE thread) and they're not as sturdy and not as much fun to wear, and ratbastid doesn't like them as much. He prefers strapless. But if you're just looking for the occasional naughty dress-up, a less expensive corset might do the trick. Be warned, though, that they are generally cheaply made, with plastic instead of metal boning, and they're not really going to give you that breathless constricted feeling, if that's what she's after. If she just wants a glorified push-up bra, get thee to ebay.
Otherwise, here are some of the best online sources for corsets:
http://www.versatilecorsets.com/F_index.html
http://www.fairygothmother.com/
http://www.galleryserpentine.com/pro...23&selection=4
http://www.darkgarden.net/index.asp
http://www.deliciouscorsets.com/catalog.html
http://www.puimond.com/html/corsets.html
http://vollercorsets.com/
http://www.absolutecorsets.com
Have fun!