We went to a Big KMart in Decatur today, since I was hoping they'd have some of the extra ponies from the first two or so waves in stock--and they do; I saw Twinkleshine, Plumsweet (both bought; three more to go), Honeybuzz, Snowcatcher, Feathermay, and Flitterheart, along with the pre-traveling Mane Six. (If their stock ends at the first few waves, I'd hate to imagine how long it'll take them to catch up...)
No wedding ponies (and part of Plumsweet's tail is either too short or cut off; I don't know whose fault that is), but I guess if you're missing any G4s, Kmart would be a good place to look if you don't mind the extra $2. *looks at purse and despairs*