The PC900 suggested by the specification is still a good choice. (Well, it's discontinued, but "PC900" was just Sharp's way of spelling "H11L1", and the H11L1 is still made by many manufacturers.)

The original 6N137 works only at 5 V. LiteOn's model also works at 3.3 V, while other manufacturers offer a separate 3.3 V variant, for a higher price: HCPL-260L, FOD260L, TLP2962. If you do use the LiteOn 6N137, you should ensure that nobody tries to substitute a different 6N137 model later.

All chips above use a DIP package. The SMD variant of the PC900 is the PC400 (and the LTV-M400, if LiteOn gets ever around to producing it). There are also 3.3 V SMD variants of the 6N137: ACPL-M60L, ACPL-M62L, FODM8061, PS9124, TLP2362. And nowadays, there are also many chips with a push/pull output: ACPL-M61L, FODM8071, TLP2361. (Currently, the TLP2361 is the cheapest SO-5 optocoupler suitable for MIDI.)