I just have a hydraulic bottle jack that fits under the seat as my jack was missing too.
Some of the parts vendors that still sell NOS/used parts and they sometimes have the jacks for sale for non-outrageous prices if you keep any eye out... or call them on the phone. They are not always listed on the site when they have them.
I tried finding an original style jack several years ago but at the time it was just a hassle... but they are still out there if you want one.
-W
