There's a house in Harbourtown West with a green Traffikrab and a guy who will give you a free bootleg. This bootleg is tied to your save's random seed, which is set when you start a new save file. After you beat the game once, you get the option to set the random seed of any new custom game. Below is a table with some seeds I tried and the associated free bootleg for each seed.

Some highlights:
Seed 8206 gives an Astral Traffikrab.
All 14 elements are given between seeds 0 and 27.

There appears to be some sort of pattern, since the same species appears for many seeds in a row before switching to another species. I'm not sure how easy it would be to reverse-engineer, but please post in the comments if you figure anything out. One thing that could be true is that if you're hunting for a specific bootleg, once you find a seed with the right species there's a good chance all 14 elemental types are in nearby seeds. Feel free to post other combinations in the comments!