Jim,
If your filter is clogged something needs to be done before the vacuum pull damages something vital.
I guess I would probably improvise as best I could if I could not come up with a new filter. So if that is the best plan at hand carefully remove the filter trying not to disturb the dirt until you can duct tape over the end inlet opening. Once the inlet is sealed tap it gently on a picnic table or some flat surface to dislodge what will give way easily. In lieu of being able to blow off the outside gently with an air gun using low pressure air at a low angle to the pleats, I would spray it with water at a low angle (not perpendicular) using a spray pattern (not stream) and see how cleanly it can be rinsed out. Then let it dry and remove the tape. If you think you might have gotten any dirt inside while removing it you could run a vacuum wand in the center as best you can.
If you can take it into a semi darkened room and aim a flashlight in the center toward the pleats and inspect all of the exterior pleats for any light breaches. If you have any light breaches, indicating damage or a hole, order another filter and wait for it.
Once you pass the inspection and your happy with the result then seat it well and put it back in service, reset the vacuum guage and let us know how it does with the vacuum level improvement. This is something I would do in your situation exercising common sense and care. It may not be for you if you would not have some comfort level with this type work.
This procedure would not provide you with a clean filter, just an unclogged filter, that would allow you to proceed until it clogs again or until you can meet up with a new filter while on the road. If anyone has a significant concern or a better improvised method please chime in....
Later Ed