I've noticed that even ebay and thrift stores tend to be the same price at the store, or similar to it. I just buy mine new at Target. But not from the toy area.
You know those pegs of impulse buys they have, right by the registers? The very bottom has some bagged toys, including G4 ponies, for $2.99. Been awhile since I bought any (I've yet to work through the last batch I bought), but last I looked a week or two ago, they still had them despite the newer molds being out. Just be forwarned, they have more obvious mold lines on them, so you might need to clean them up a bit that way before using them.