While I usually agree with you, on this one I think you're being a bit too obstructionist to the players.
For one thing, 250 g.p. value
does equal 250 g.p. for sale. If it didn't, the spell write-up would list a different value. The 250 is there as a flag to both DM and player that says nothing more than "To cast this spell, you gotta spend 250 clams on a tuning fork".
For another, given the spell is fairly common, one would think the tuning forks would be fairly common as well. Clerics often have temples, and said temples might even have gathered a collection of tuning forks for different planes over the years, to be loaned out to their Clerics as and when required.
Or, you could just drop the whole needs-a-tuning-fork idea completely; which is how I'd deal with it.