Where can I get hold of a second hand standing frame

The NHS has provided a standing frame for my 5 yr old son's use at school but won't provide one for home as well and our physio (private) really thinks he needs extra time upright & weight bearing to reduce some of the tightness in his legs and to prevent his hips getting even worse - so we'd liek to buy one but they're obviously really expensive new so wondered if there are any places to look for anyone selling off one their child might have outgronw.
Any ideas / suggesitons really appreciated.
Thanks