You can buy cheap hardware store primer, but I'd avoid it.  If you want to use primer in a spray can, use Tamiya's Fine primers.  They are expensive, but their performance for scale models cannot be beat.  If you want to airbrush the primer on, I'd use Tamiya liquid surface primer or Mr. Surfacer, appropriately thinned.

I use Tamiya basic putty for medium size gaps, Tamiya epoxy putty for larger gaps, or Tamiya liquid surface primer or Mr. Surfacer for small gaps.  I also sometimes use the spare styrene and liquid cement trick for some gaps.

You can use superglue to fill a gap, but it's very hard to sand or shape and you'll end up tearing up the soft plastic around the superglue before you make a dent in the superglue.