Talk to a specialist or work out what you want (size, brand etc).
Then look at buying a new set (expensive) or look at where you can get them from. Old Alfa's, 18RGs, 2TGs spring to mind, or even better, hang out with the rally crowd and see who's selling what off their L16/18/20.
Make sure you know someone who can rebuild them before you buy second hand though. I know Rallystanza recommends a carby place in Preston who know their stuff.
Or just go quad throttles.
Hen