I actually work for a sister company of Airdefense and I have to tell you there stuff is really sweet. Our offices used to be next to each other and we got to play with a few of their toys when we were back at home office for meetings. All I can say is it's well worth the price
Airdefense is also very well respected in the network security community. Do a search on the security focus archives, for instance.
The other two suggestions about checking for MAC addrs of common WAP hardware and using nmap to scan for known WAPs are very good (although frighteningly tedious for 200 WAN sites). You will probably want to setup software to monitor ARP tables at each site (from routers and managed switches) and report back any matches. You'll need some type of central database of MAC addrs that you can update and push out to each site.