I have them in all sizes from 4 oz to half gallon. I can find them at Long's Drugs right now for a good price. They carry them year round but right now they have an entire aisle devoted to them. I can also find them at the hardware store, they have an entire canning section with pressure canners, water canners, lids and jars. If you're going to buy them, I recommend getting wide-mouth jars in all the sizes. They're easier to fill and clean than the narrow-mouth jars. You won't really have that option if you're not buying them new though.
Asking friends and family, freecycle, thrift stores are all great places to get them for cheap. Like a pp mentioned, mayo used to come in glass jars that you could can in. TJ's canola mayo still does. Most other mayos come in plastic now.
But if you buy any jams, jellies, pickles, olives, PB, spaghetti sauce, anything like that in a standard sized jar (most of them don't come that way, but some do), then the jars are reusable. When I was a kid, we used to seal our jam with wax rather than 2 part lids (which meant we could use any jar), but since I don't use a ton of sugar in my jams now, I can't safely do that. If you have a dishwasher, once you've cleaned any dried on crud off them, run them through to make sure they're really clean. Then inspect them for chips and cracks before putting anything in them. If they are chipped or cracked, depending on the location, they could be used for dry storage, or they might need to go right into recycling.
For jams, I usually use 8 or 12 oz jars. If I'm making jams as gifts (where I'll give a couple different flavors), that's where I'll use the 4 oz jars. Pints are what I use for things we might only use occasionally, like pickled beets or relish, even tomatoes which don't keep well in the fridge... a half-pint is too small, but a quart is too much. Quarts are used for things we go through (and keep well in the fridge). Like canned fruits, pickled asparagus/green beans, rendered fat, etc. I don't use the half gallons for canning at this point... but I know my grandma used them for juices.