Well, first you don't want a reversing ring, you want a macro coupling ring.
A reversing ring allows you to reverse a single lens. One side has a lens mount, the other side has filter threads.
A macro coupling ring has male filter threads on both sides, so you can screw one lens onto another (facing each other).
The trick will be finding a macro coupling ring that goes between sizes. Normally they have the same size threads on each side. If you want to couple between lenses with different threads you'll also need the appropriate step-up or step-down fiilter adapters.
Adorama carry all sorts of step-up, step-down and macro coupling rings. You'll have to dig around to find the sizes you need. Here are some examples to start you off.
* 62mm-62mm macro coupling ring
* Step up ring
* Step down ring
Your best bet would probably be to start with two prime lenses, such as a 100mm prime with a 50mm prime reversed. That will give you 2x. Mounting a wideangle lens reversed on a telephoto zoom might give you some problems. Maybe vignetting, maybe some quality loss. You don't have to reverse a Canon EOS lens of course. You can use any lens, including lenses with a mechanical iris which you can adjust.