What is the difference in the 2n3904s? I am messing with a circuit and it as a 2n3904-338 in it. It is hard to come up with one of those exactly. Can I just use another 2n3904? Also, What is the best place to get small parts like this, a project box, resistors etc?

 

The -338 may just be the manufacturer's year and week that it was produced.

Hard to say without knowing exactly which manufacturer made it.

Generally speaking, you should be able to substitute one 2N3904 transistor for another if the circuit was designed correctly. No two transistors are exactly alike, but a properly designed circuit will function properly with a transistor that meets manufacturer's specifications.

As far as where to get them - if you don't have any spares onhand in the "ye olde junque box", you may have to order them. Don't know where you are on the planet, but if you're in the States, Digikey is a good place to get parts from. They will ship SMALL orders via USPS 1st Class mail, which will save you money.

They don't have a minimum order requirement, and are "hobbyist friendly".