any garage who services cars will have them - they buy a pump and change oil that way as cheaper than small containers. They are usually happy to give them away as long as they're completely empty
We get ours from the local paint factory - and if you ask very nicely - they also give us free any battered tins of bright coloured paint to paint the jumps with as well. Various sizes from small plastic paint points (ideal to start the babies off with) right up to the big 3' size ones. They have big trouble getting rid of them and have told us to tell friends who want them to take them away. In return, we just give them a bottle of plonk. (By the way, the paint factory is in Soham (between Newmarket and Ely).