I've improvised ...
rubber bands
string
anything else that looked like it was about the right size ...
You do have to be careful to get the pressure just right. The right constriction causes an intense hard-on that lasts a bit longer and feels a bit more sensitive. Who needs viagra??
Here's something for you to try: take a piece of lace or something soft, and wrap it around your scrotum, so your balls are extended down away from the shaft of your dick ... not too tight, mind, otherwise you could cause an injury. Key is to make that extension, firmly but without pain. When you cum it causes an extra rush ... very intense ...
Apparently that's supposed to be an advanced pro trick, to pull your balls down just before orgasm, but I never found a pro who knew it!