I am new to this list, and i would like to ask if anyone knows where
I can get a 5 X 5 X 5 Rubik's Cube.

Can't help with this...unless you're in Montreal, in which case you
might give Valet de Coeur a try. I think that's where I got mine.

I have already mastered the 3 and 4 cubes, and I don't think that the
5 will be much different.

You're right; it won't. If you can handle the 3-cube and 4-cube,
you've got all the necessary skills. About the only further challenge
is the supergroup, where (for example) each face has a picture on it,
so that the inner face cubies have to be positioned (and oriented, for
the center cubie on an odd-sized cube) correctly. I have never seen
such a thing for cubes larger than 3x3x3, but I also have never
bothered really looking for one.

der Mouse

mouse@collatz.mcrcim.mcgill.edu