Pretty much any of the Japanese ones. The recent bookcase re-issue, goodbye Megatron and New Year's Megatron might be what you're looking for. Or maybe the e-hobby black Megatron And I think he shoots little rubber bullets, not pellets. edit: Oh, and the Encore edition, forgot about that one, and that may be the easiest to find. There was also a DOTM movie megatron + G1 Megatron double-pack in Japan you could look for.
Factoring for shipping? I honestly don't know. Maybe the encore set. Check the junkion exchange, especially if you can get one loose.