For a good stretchy underwear for a wedgie that can pull really high and also gets in deep, the key is to have athletic-type underwear for stretch, but isn't compression material which prevents the actual wedgie. In a store, the thing to look for would be spandex. The fabric doesn't have to be pulled hard to stretch. Champion C9 Men's Power Cool boxer briefs are pretty good, available, and not expensive.
For hanging wedgies, it's more about the setup than the underwear because typically it's putting a lot of stress in a small part of the fabric, causing it to rip. The set up is also really important for safety. I wouldn't recommend anyone do a hanging wedgie, because it's a lot of weight on a small area of the body and because if done improperly it could be easy to get stuck, which is a very serious risk. But if it's going to happen, it should be done as safely as possible. One of the safest is the standing-on-a-rope method that was mentioned on another site.
The steps for the standing-on-a-rope method are:
1. Obtain a tennis ball and both a carabiner and rope that are strong enough to support the weight of the person.
2. With the underwear off, put the tennis ball inside the back of the underwear against the fabric just below where the tag is.
3. Pinch the fabric so that the tennis ball is in a little pocket, and attach the carabiner around it on the outside of the underwear.
4. The result is a pair of underwear with a carabiner attached on the outside without having to make a hole. The tennis ball keeps the carabiner from just falling off outside the underwear, and the carabiner keeps the pocket for the tennis ball from falling loose on the inside. It's now a pair of underwear with a secure loop right where it's wanted.
5. Attach one end of the rope securely to the carabiner with a non-slip knot and throw the length of the rope over something strong enough to support far more than the weight of the person.
6. Create a loop in the rope with another non-slip knot. This is what the person will stand on. It may take a few tries to place the loop correctly so that the length of the rope is right.
7. Put on the underwear, then slowly and with steady balance step onto the loop so that neither foot is touching the ground.
8. The weight pulls down the rope on one end, which pulls up the rope on the other end, which is conveniently attached to the carabiner. Because nothing is acting as a hook or spike on the fabric, it's less likely to rip.
This might not be the classic image of a hanging wedgie as having no way out, but it's every bit as painful. Physics doesn't care that the end of the rope is attached to the person's foot instead of the floor, the full weight is still being supported in the wedgie. The advantage is that it's much safer because it's easy to get out by simply stepping out of the loop and extending a leg.
Okay with: messy, all around, melvins, spanking, swirlies
Hard Limits: No public (under clothes for short trip is ok), no violence, no pics, no scat