I know that there are plenty of 'substitute' vaginas available at the porn shops, some with built-in vibrators, but here's a technique I dreamed up long before such things could be purchased:
Get a jar that will fit snugly-but not too snugly-over your erect penis. I always used olive jars, but there are others that will work. Empty it, then get a glass cutter from the hardware store and cut the closed end out of it by scoring around the outside of the base. A tap from the inside ought to knock the bottom out. (Leaving the bottom in creates too much suction; if you want some suction, maybe you can find a way to make a smaller hole in the bottom or cut away just part of it.) Even if the result is a bit jagged, don't worry. Just take a metal file and round off the edges so you won't get cut.
Get one of those vibrators that straps onto the back of your hand, lubricate the inside of the jar and put it under the straps. Put the whole arrangement on a pillow, with a towel under it to keep from messing the pillow up. Lie on your side and insert your erection into the jar and then turn on the vibrator. (You may want to put another pillow on top, also, to hold things in place.)
You'll be amazed at how well this works. I think the hardness of the glass jar does a better job of transmitting the vibrations to your penis than a soft plastic imitation vagina does. I used this technique for years and years when I was in college and living in my own room at home. Kept me quite well satisfied!