Are you talking about dresses? Simple lingerie (full slip, half slip, camisole), things like garters and stockings? Or nightgowns/baby dolls??
Where to buy is easy - the same places a real girl buys her stuff at!
If it's personal things, you can invest in quality, like Victoria's Secret, or even Fredericks. JCPenny has some nice things at reasonable prices too.
For plus sizes there's Torrid, or Lane Bryant. Shop the bargain-bins.
If you are building a wardrobe, online is a good place to find used stuff, as are thrift stores.