DLR goes straight to it, got its own station. I found it alright and Im a public transport novice. Presumably only a problem if you have to go right across London.
Have a look at Systemq, they have some decent stuff. IIRC you will need proof of trade to get prices off them.
But other than that IME most ready to fit coverts are too easy to spot and you're better off knocking something up yourself.