Cherax quads (12"), and depending on your area (I can get them easily here, but up one state it's impossible) Cherax cainii and Cherax tenuimanus (16").
Cherax quads are relatively cheap and they show up in most LFS except for states that they've been banned in. Cherax cainii and teruimanus show up in a lot of places but are easier to purchase online. Everything else is relatively hard to find or have imported, or not going to fall under "the largest type".