First of all, aluminum shafts are seriously expensive. You need to decide whether or not to spend the $$. I don't spend much time under my car looking up at the underpinnings, so steel is fine with me. I guess you could find a shaft from a donor car, but like Bob said, any driveshaft shop can make what you need. Most shops would rather work with new tube anyhow, and by the time you hunt down an old shaft, clean it up, have them cut off the junk, re-weld and re-balance, I've found the cost isn't much different than a whole new shaft.

Either way, check with the shop FIRST to made sure how they want you to measure the shaft length. Different shops have different methods. You may want to get a yoke from them first so you can stick it in the trans and measure the length. Also, make sure that you have enough slack on the trans end when you measure. You can't shove the yoke into the trans until it bottoms out. You need some slack to compensate for the effective change in length of the driveshaft as the rear axle moves up and down. To get driveshaft length, I usually push the yoke all the way in, measure for the shaft, then deduct an inch.

Tell them what rear-end you have, and what transmission, and they'll supply the yoke, u-joints, welding and balancing. They'll even paint it with some crappy, low-grade rattle can paint at no extra charge.

Also, bear in mind that there are many, many different sizes of u-joints. The trans end won't be a problem, since you're buying the yoke with it. However, make sure you know what u-joint goes on the differential end. I tend to use 1350 series whereever possible, because they are genuine hard-core strong.