I got a bumbo full price from mothercare, and a tray for full price on ebay. I would have got one off ebay buut with the postage it wasnt much cheaper than buying one new, andthere were none up for collection by us.
You could always try bargain pages or local papers? I just think that yeah I spent £40 on it with the tray but it'll resell on ebay so I don't mind so much cos I'll get most of the money back.