You can make your own if you want to - if you also have the air filter housing. Clean up the mating faces of both the carb and the housing, then wrap both tightly in cling film. Spread a good quality silicone sealant (I would recommend Dow Corning 791 black as a starting point, about a fiver from Toolstation/Screwfix) around the mating face of the carb (on top of the cling film) and then gently but firmly press the housing onto the carb. Give it a GENTLE twist back and forth about an inch, JUST ONCE to spread the sealant without squeezing it out all over the place. Don't ram the housing down hard, don't bolt it up tight, just let it rest gently in place overnight.
Next day, preferably a full 24 hours later, even better if you can leave it a weekend, take it all apart and peel off the cling film. The sealant may still be a bit tacky, so leave it another day if you can to cure fully, then you can trim off any rough edges, and there's your gasket.
These sealants cure by giving off acetic acid vapour (vinegar) so the looser you can leave the housing on the carb the better, as this will leave a wider un-sealed edge for the vapour to evaporate through. Once the outer edges of the gasket have skinned enough to allow handling you can remove the housing, but the core of the sealant will still be liquid-ish, so leave as long as you can for full evaporation/curing. The longer you can leave it the more robust the gasket and the more easily it can be handled.
These sealants are also designed to be ADHESIVES, so do NOT be tempted just to use sealant directly on the carb/housing, or next time you want to remove the housing you'll find yourself cutting it off with a Stanley knife!
