Puffers do get bored. They are rather intelligent as far as fish go and prone to glass surfing when they run out of things to keep them occupied. With SAP's it isn't so much that they are one of the few to do good in groups. GSP's, F8's, Dwarfs, etc. all do perfectly well in groups, assuming the tank is large enough. SAP's are one of the few to actually prefer groups. 3+ should work fine. For that size tank, probably 3.
One of the problems with fish is that species substitution is not all that common when it comes to groupings. You'll sometimes see it, but usually not enough to rely on it for substituting for a lack of the right species. Unfortunately it is going to be even less likely with a SAP, since you aren't going to find another species with similar looks and behavior.
If you haven't already, see if you LFS can special order them. They may not carry them but they may still be able to order them in. Otherwise another LFS or online will be the only other options.
If it were an F8 or GSP I'd say throw a bunch of feeder guppies in to keep it busy, but SAP's are often too nice for that. If possible though you could try that, to give it something to do and chase (if it even cares about other fish). The other method often used is the "throw in a ton of snails to keep it occupied on hunting"-method. Unfortunately SAP's decimate snails faster then the other puffers so that may not be realistic.