The best inflatable pools for mud wrestling are the kind that have 3, or at the very least 2 inflatable rings. These pools tend to hold their shape better so the mud won't spill over the edges if a wrestler lands on the side of the pool. Also if one of the rings accidentally gets popped you still have a ring or two holding the mud in, to keep the action going.
Some good pool options are listed below. The bigger the pool the better, as it will give the wrestlers more room to throw each other around, and more than two people can wrestle at once. Larger pools do cost a bit more though, and require more mud, so you can select a pool based on your budget. Don't worry too much, we have had a great time wrestling in some fairly small pools.