Probably your simplest solution is to find out where old bikes are sold after an overhaul/service in your
local area . Their bikes are usually donated
So not bike shops as such , but organizations who give employment to disadvantaged people etc
There are about 3 or 4 in my area , with stock always changing , and prices are reasonable
bearing in mind that the bikes have been serviced etc
Otherwise try the Gumtree route and have a look around , your not going to have to
spend a great deal , and should have loads of fun !
Good luck