How do I get from Red Deer to Jasper?

Use another bus company. When Greyhound stops driving routes, others will adjust and hopefully add routes, but it looks like there are already alternatives.

e.g. you can take Red Arrow to Edmonton, then take a VIA Rail train or a (more expensive) Sundog shuttle from there to Jasper.

The first link should be: www.redarrow.ca/

Never been to either so just guessing 5 Ways To Get There

This might be fun to investigate. Ride Sharing in Red Deer.
And you’d meet someone. :-)

Getting to Edmonton should be OK. From there, if Greyhound isn’t running and no bus company is operating you can try this rideshare group