I had similar requirements recently.
Ive always found ebay a good source. Cheap, folding and lightweight don’t always necessarily go hand in hand but my advice is:-
1. Set a budget and stick to it
2. Confirm the weight as some ‘lightweight’ ones are not.
3. Be patient and bide your time- you will find one.
My guess is that you could get one between £150-£300 in excellent condition.