I love this thread since I'm having the same issues. And I've never been able to figure out why my size in everything else changes when I lose weight, but undies don't.
When I do get to my goal weight, I plan on buying a lot of new stuff for myself from Victoria Secrets and a few lingerie websites I've found online. I've been making wishlists and picking out everything that I've always wanted to wear but haven't been able to. Because Lord knows, I'm so over having to wear ugly underwear just because.