VEX Mecanum wheels are work decently, but I've heard (and seen) 6 inch Andy Mark work pretty well. If quality isn't as much as an issue, and your just playing around with the wheels, maybe you could 3D print wheels? It's a bit time consuming, but if your team has a 3D printer and it's for experimental purposes, it might be something to consider. Also, it will be cheaper
If your interested, message me and I can send you a file.