If you buy a 2nd hand cot bed, better buy a new mattress (accidents do happen and old mattresses are not v. safe for babies).
For 2nd hand baby items in general you could try ebay.co.uk or nct.org.uk (nearly new sales in your local area). IKEA has very cheap new cots.
If you're looking for something longer lasting, go for a cot which converts into a junior bed, size 70cm x 140cm. Things to look out for are also removable bars, variable heights or lowering sides, wheels etc. There used to be an excellent bargain at jako-o.de (English website available, 5 pounds delivery charge, 320 Euro complete cot + conversion kit with mattress, linen etc - currently sold out but might come in again so worth asking).
If you are looking for second hand baby/childrens stuff worth visiting the FARA Charity Shop in Tatchbrook St, Pimlico, which is devoted to chidlrens stuff. (Get off the C10 bus at Pimlico Tube and walk straight up.) I did not look to see if they had a cot, but they had a row of nice looking prams.
NCT sales are worth going to. Its a few years since I have been, but the Clapham ones were usually better than the Dulwich ones. (If you live in Nappy Valley and aspire to being a yummy mummy you need the right kit.) The national NCT can give you the contacts. There also used to be a sale in Dulwich organised by, I think, TAMBA. (Twins and Multiple Birth Assn.) Members had first pick and then the sale was open to all.
You can also post on the Families website. (The site linked to a collection of magazines including Families South West and Families South East.) For whatever reason, I never in my years of going to playgroups etc came across Families South East, but Families South West seems to be pretty well distributed and has quite a lot of useful stuff, though with a very Clapham/Battersea/Wandsworth focus. But then I have always aspired to being Wandsworth woman, complete with large car and tennis club membership.