I think I sent this to you in an email, but ...
Can't help on used ones other than to suggest watching ebay. However there are some new ones that are readily available:
$ 795.99 - Generic 10" filter tower
$ 974.54 - Coltri Sub 14" filter tower (also has a water trap at base)
$ 895.99 - Generic 27" filter tower
$ 793.41 - Generic 1# filter cartridge housing
$ 849.90 - Generic 2# filter cartridge housing
$ 875.52 - Generic 3# filter cartridge housing
$1,224.22 - Generic 5# filter cartridge housing
If you pack your own, the price is a whole lot cheaper for filter medium as the cartridges range from about $50 to well over $250 per change!
I know that these are out of your budget, but just thought I would toss it out for reference for when you are negotiating with a used seller.
Good hunting!!