You pretty much have no choice except to join a local Realtor Association. If you do not, you will not be able to get a lockbox to show properties. Also you will not be able to list properties in the MLS.
I'm in Tampa and our rules may be slightly different but we do have two or three local Realtor Associations and there are two MLS systems. I would investigate the MLS systems associated with the Realtor Assoc before joining to see which one has the best MLS system.
I think eventually Realtors will wise up and realize local Realtor Associations are an unecessary expense. We're all required to be members of NAR so we should also just have a simple nationwide MLS system and eliminate all of the middlemen (local boards) to lower our costs.