There are many places to get parts for HSMM radios. I am incuding a list of some that I have bought from.
FAB-Corp Good product and fast shipping. They have anttenas, boxes, adapters and connectors.
Radio Shack Masts and tripods along with cables and other small parts.
EBAY Almost any part for this system you would need. Be careful and ask lots of questions.
MFJ MFJ-1800 WI-FI Antenna and many other fun things!
These are only a few of the many places to get started with HSMM.