You're not leaving yourself a lot of options. There's only two ways to do this: distributed, or centralized. There's no third model: either the server is local, or it's remote. You're going to either be sending your AAA traffic over the WAN, or you're going to be putting AAA services in each office.
If you need AAA but don't absolutely have to have ACS, you might consider using a different (free or less expensive) radius server, or use the radius server in the access point itself.
-Gabriel
