Getting a puffer to puff just for visual pleasure is never a good thing. Some do it because they are frightened, scared, warning/scaring others off its territory.
If you go to your LFS
, they will cost $20-$75 in Washington State. The rarer and bigger ones will cost more.
The smaller species of puffers don't really look like the typical round puffer (i.e. Valentini or Papuan Toby puffer). If you want something round, then you could always get a Green Spotted Puffer because they will eventually become marine puffers and very cheap (~$10). They stay somewhat small (up to ~6.5inch long) and fun to watch (all puffs are fun to watch).
Please make sure your tank is at least 40 gallons for whatever puff you choose.