We have made then in the past out of wire mesh from B&Q.
You have to be really carefull about sharp edges and make sure they are all tucked safly on the inside. Also have to make sure the mesh is really small, Ginge kept getting in to one and then got stuck trying to get back out but 5mm should be fine even with yearlings.
Reptiles-ink on the dark side £7 a piece, may do a deal for bulk, and you could collect them at the York Reptile Night (or he will post but that hikes up the cost)
cornsnakes, ratsnakes, bullsnakes, boas and day geckos