http://www.justengines.unseen.org/carbs.htm

Is a reliable UK supplier.

However a Want ad in the buy/sell area may well bring a free carb as many people in the USA appeared to have replaced the original carb with an [ expensive replacement ] . The early carbs can be problematic but I have had good results with all my MDS engines and almost every one I have set up for others. Just follow the instructions.