I'm trying to mount a pickup in a guitar. The screws supplied by the
manufacturer are 3-48x1.25" and are too short.

There are few fastener companies selling 3-48 screws to begin with but
I can't find anyone selling a 1.5" length.

Anyone know where I can get these?

Otherwise, I will have to buy a 4-40tpi tap and switch to 4-40 screws.
Those I can find in 1.5" lengths.

Jaz