I know others here have bought them in the past but I don't know where they got them, and I'm not sure they are available in the UK.
Just be aware that often these are very tiny when bought, expensive, and are difficult to care for. Many very experienced growers struggle with them. Our conditions in the UK are very different to those in the swamps where they grow in the USA and as such unless you can recreate those conditions you are unlikely to be successful with them.
It's a shame because I think they are wonderful, but personally I don't think I could grow one.