I've replaced a broken pull (just the tab part that lets you get a grip on the slider) with a small split ring, similar to what are used for key rings only smaller. These are available in hardware or home improvement stores, where the key rings are shelved. I had a pair of boots with such a split ring that I wore that way for three years, pulling the zipper up and down at least once a day each direction.
If the whole slider is failing, you might want to contact an outdoor gear repairer (yes, there are people who repair things like tents, sleeping bags, and backpacks professionally) to see what it would cost to have the complete zipper replaced. It's unlikely to be economical relative to buying a new backpack, but if there's sentimental value there, money may be a lesser concern.