6N138 is a JEDEC part code so they all have the same basic specs. Just grab one in the right package (DIP/PDIP for through hole, not SMD).
Note that 6N138 appears in a lot of internet schematics, but strictly speaking it’s not fast enough to match the MIDI spec requirements, even if that probably doesn’t matter for normal use.
(I use 6N137 instead, which is much faster, has a logic gate output instead of a darlington transistor which is nice since you’re connecting it to a logic gate input, and is readily available from Tayda and others. See schematics here.)