My preferred source of Mychorrizal fungi is Piranha. I use pretty much all Advanced Nutrients for everything else so that's part of the reason, but mostly I just feel it's the best.
There's a lot of different species of mychorrizae so you not only want to make sure you're getting the right kinds (not all colonize all kinds of plants) but you also want to get as many of the kinds that work with your plants as possible since they each play slightly different roles.
Perhaps more importantly though, you want to make sure the mychorrizal fungi you get are still alive. Fungi are hardy little things, but there's still lots of ways for them to get killed so you need to be sure of the quality you're paying for. That's the main reasons I go with Piranha. I know it's got eight species of trichoderma and 18 species of endomycorrhizal and ectomycorrhizal fungi of the right species for my plants and I know that what I'm giving them is still alive and not just the powdery carcasses of dead fungal spores.