Mine is pretty dried out too...not so much cracked, but discolored and looks like hell...
Probably a dealer only type of part...Lots of aftermarket antennae are available, but the rubber stub thing is probably Mazda only...I'd look into that soon if it is noticeably cracked (making me think i should do mine now too), as i'm sure it'll start letting water in...which can't be good for the head liner...
as a quick temp fix you could find some black sealant/gasket maker to run around the lip touching the roof paint...that would at least water-proof it pretty well for the time being.