Many of us build our instruments rather than buy them. It can be a lot cheaper that way.
Go find a piece of glass the diameter you want to make a mirror with and then cut the mirror blank round out of it and then grind and polish the piece of glass and finally put a silver deposit on it with any number of different methods that are available. Bare silver will last from 3 months to several years, depending upon the atmosphere around you.
Go read up on John Dobson for making mirrors on the cheap. At one time he was a monk (an asian religion) and made mirrors for essentially nothing but his time and effort. He has build some huge mirrors (over 24" or 600mm in diameter) so you CAN do it! Having access to Silicon Carbide sand does help (sandblasters use it for blasting some stuff) and learning how to titrate the sand into it`s various sizes helps also.
You might want to stop by my website http://webu.wigloo.com/bobmay for one insight into grinding and polishing a mirror. There`s a lot more websites out there that deal with grinding and polishing and each one of them has strong points and weak points so go find!