I'm not aware of any other 2 part epoxy based finishes that are similar to Thunderbird. Someone may offer something similar, but I personally don't know of any.
Fullerplast would be a good option for an alternative. It's not quite as tough as Thunderbird epoxy, but it's tough enough.
Some use ML Cambell's Krystal conversion varnish. I used it for a while, but personally don't like it that well for bow finish.
If you like a gloss finish, 2 part automotive urethane is tough to beat. I prefer using it to anything else, however most guys don't like a gloss finish on their hunting bow.
Only about 1 in 10 customers order a bow with gloss finish.
For a durable low luster alternative to Thunderbird, I've been looking into various marine finishes but I haven't actually tried any yet, so I can't give any recomendations on those. They can be a little pricey and might not be an economical choice if you only build a few bows a year.
One thing about finish is that there's always pro's and con's with any of them. What works good for one guy might not be the best choice for another.