Try the dealers listed in the resource section for the gaskets. Ike's Bikes is highly recomended. In addition to replacing the filter screen it is highly recomended to add an in line fule filter. You can get an in line filter at any auto supply store. You can probably get an in line filter kit, or, do what I did, just look through the selection of fule filters and choose one that will hook up with your fule line.
Also, replace the old spark plug, a fresh one will save you a lot of effort looking for problems elsewhere. Even a plug that looks good can be a bit off.