To answer the title question, ILSCO's "Mac Block Connector" can connect any Cu or Al wires up to #6 at affordable prices.
The conductors need to be big enough.
For 50A, you need any of the following:
- Copper NM or UF cable: 6 AWG CU (55A actually)
- Copper (any other type of) cable or wire with 75C terminations: 8 AWG CU
- Aluminum (any other type) with 75C terminations: 6 AWG AL
For feeder of this size, aluminum is not a problem as long as the terminations are rated for it. (that last was the crux of the problem you might have heard about. It also helps to set screw torques correctly, but that's true on any connection.)
For 50A circuits, ground must be #10. Unless...
A 30-60A circuit needs a safety ground wire of #10 copper or #8 aluminum. Any competent wire manufacturer will make cable with a ground of appropriate size.
The only exception is if you choose conductors larger than are required. If so, the ground must get a proportional "size bump" as well. This is a problem you will never have in multi-conductor cable, since not only do they bump ground sizes in proportion, but they are typically overkill anyway.