I found that anything pre-made is disproportionately expensive because it's all retro gear aimed at studios, and not that common. There's not much to them, it's not a matter of converting protocols or anything, just some chips to interface the electrical signals while protecting both sides. Unfortunately I couldn't find anyone that sells new ones pre-made, so I ended up building one based on a post here, and I found all of the parts could be found at Jaycar for less than $20 (definitely less than $10 if you were buying in bulk). The most expensive part is the 5-pin DIN connecters, so I went with 3 of them, one each of MIDI in/out/thru.
If you're okay at soldering you should be fine to build one, if you want I can post some photos of how I wired it up on prototyping board.