How often (or under what conditions) does Rowf's Badge Shop in Toad Town get new or different badges? I'd hate to miss a great badge because I didn't visit the shop at the right time.
The badges won't change before you buy them. After that, the empty slots will be filled up when you re-enter town (if new badges are available, usually after a chapter finishes). So, don't worry, just buy the ones you don't actually need later when you got some spare coins.