I wonder how many folks have learned that when the slider on your zipper wears out, they usually aren't too hard to replace yourself. I called a place and they said they would want $30-60 to replace a slider on my tent zipper. So I found that
ZRK Enterprises, the zipper rescue folks, sell kits for like $10 with a variety of sliders, as well as instructions. And they are happy to talk to you by email or phone if you need more help. They are in Ashland. You can also get their outdoor kit through Campmor. One kit can do many repairs if each one needs a different size or type of slider.
You have to remove the zipper stop or box, and replace the slider. Sometimes they slide right on, sometimes it is finicky and you have to fiddle with it for a while to get it on. But my tent works fine now, and I still have more sliders for other future zipper repairs.
If the zipper itself is worn out, that's a whole nother story.