Electronic – ny formula to get number of windings for a 50W voice coil of a speaker

That has been a subject of debate for decades. Should the speakers be overrated or the amplifier?

1. If the speakers are overrated the amplifier might be strained at high volume levels and produce distortion (clipping) that not only sounds bad, it is bad for the speakers as they faithfully try to reproduce the distortion.
2. If the amplifier is overrated (but not by an outrageous amount like 10:1), then you get clean sound until the speakers reach their limit. The woofers may pop against their frame and the sound becomes distorted. This is a warning to back off the sound level or risk burning up a speaker.
3. Because there is seldom anything like a perfect match between speaker wattage ratings and that of the amplifier, the better match is to have an amplifier that is a little over rated, and be careful with the volume control. This way the amp is never strained or distorted, giving you clean sound up to the maximum volume the speaker can handle.
4. It is easy to burn up 15 watt speakers, but they are often very efficient and will be very loud at a safe maximum volume. A 50 watt amplifier will drive these speakers with ease and not distort from working too hard.
   

EDIT: For better mechanical range of the volume control, look for a gain adjust control on the amp or buy 2 RCA attenuators from most any stereo installation shop(car or home). Ones with -3db rating will cut signal level to 70.7%. Those with a -6db rating will cut sound level by 50%. Any of these options will allow you to use more of the mechanical 'rotation degrees' of the volume control before you reach the limit of the speakers. (To avoid confusion, I am referring to signal level, not power levels.)

"Level" (voltage) isn't likely the issue here — while the voltage can vary much more widely than line level, it is unlikely for the voltage in any modern speaker system to exceed the voltage rating of the wire insulation, and it may not even be particularly higher than line level connections. The difference is in the load impedance (likely 2-8 Ω rather than “high”) and therefore the current (higher). That does mean you're right to want heavier wires, though a regular cable might work fine.

A common source of larger gauge three-conductor cable that comes to mind would be grounded AC line cords. The insulation will be overkill, though (both for the higher voltage and for wear resistance/safety), which will make the cable less flexible and larger diameter than otherwise.

You could make your own cable by using one-and-a-half lengths of standard speaker wire and twisting them tightly together, letting them relax again, then applying braided or spiral cable sleeving along the length, or heat-shrink tubing at key points.

You may also want to reconsider using TS/TRS connectors, as they can short together the contacts if partially unplugged, which might be worse for a speaker amplifier (originally intended to be entirely internal to the guitar amp, too) than other devices more usually using such connectors which are designed to tolerate the temporary short.

As to why no "stereo speaker cable" exists, it is probably just that left and right speakers are put in separate places so two separate cables are needed, and using a partial run of three-conductor cable would be more complex.