Is it possible to get replacement alarm fobs for aftermarket alarms?

Who would be the person to go to, the company who installed the alarm or the manufacturers?

I haven't got a big name alarm like a Clifford or a Toad, but I really need to get some replacement fobs as one has already given up the ghost, and I don't want to be left without one at all.

The alarm was installed in 1999 will this make a difference?

Thanks