Go to (all craigs list and buy a used suit)Cheapest way or cut up a old radio tire and strap to your knees.
What size are you may know someone that has a suit for $75
If you are actually wanting to drag your knee, you should have on track pants. They are tight and do not let the slider to move to the side. Also, if you are doing that kind of riding, you should be fully geared anyways. I bought A* track pants on Craigslist for $125. I would recommend going to cycle gear and buying either Frank Thomas track pants or Built pants, either should be something like $100.
Just strap 2 hard hats to each knee and your ready to go. By doing this you won't have to lean as far because the hard hat will extend off your knee about 7" thus making you an over night knee dragger.......Awesome with a touch of sarcasm
knee pads to wear under jeans are like $35 at cycle gear, it's what I wear on street rides. Very comfortable and they extend down the shin and tuck inside the boots. They don't move around either when dragging knee but that is not the purpose, they are made to protect you if you go down.