Pure Freedom, RO Man etc as already mentioned.
I will say though (and this is no way is aimed at any of the above) that be careful when looking at 2nds on eBay, as the main bulk of the cost of a working RO unit is the membrane(s), which have a lifetime.
A 450GPD system may look good on ebay for under £100, but if it needs new membranes, that could be an extra £100 on top...