What is the gasket
made of, and do you have a pair of calipers to measure its thickness? Is it an O-ring or a flat cut gasket?
Sometimes it is easier to track down the gasket material, and cut one yourself when dealing with slow retailers.
Add a little to the thickness/round up to the next available size if it is flat. If you have the old gasket, lay it on top of your sheet of material and cut it out with an exacto knife. Straight lines are easily done on a paper cutting table. Bolt holes are most easily done by laying the gasket on the part and tapping lightly with a ball end of a ball peen hammer. That is, given the part you are working with has sharp edges, and is thick enough not to be distorted.
I often find lack of parts
availability frustrating, and or the replacement just leaks
again next year. A lot of gasket materials are better quality than what manufacturers put in them... always a pursuit of profit. Grrr.
Hope that helps...