If you have a used sports equipment store like Play it Again Sports, you might find something useable. Soccer shin guards like Sixty Fiver mentioned are one possibility. You might also see if they have any baseball catcher leg guards. They usually cover shins, knees, and have stuff to go over the feet. You can cut off the parts you don't want. I've gone there looking for stuff like that sometimes for stuff to use in costumes. I picked up a set of those leg guards for about $9 for making a Predator costume. Sometimes sports stores will also have ones for field hockey. I don't know if there'd be anything comparable for paintball.
When it comes to protection, you obviously don't want to shortchange yourself. In the battle of cost vs. quality, there are times to let quality win and times where cost just gets so overblown its ridiculous. Take shades. I just picked up a pair of yellow-lensed safety glasses at Lowe's for $9 that look about the same as a set they had at my LBS for $60. Since they're making safety glasses more "fashionable" now, you can find ones that don't look like you swiped them from a science lab or construction site. I've found both types fog just about the same, so a little anti-fog stuff like Cat Cr-p will do the trick.