My Google-fu just leveled up. Look what I found:

/bind * "setactivecostume 0 0" <!-- This is your first costume slot -->

Just up the second number at the end for each extra costume you have. So 0 1, 0 2 etc.

Replace the * with the key of your choice.

Some people I am sure have found this out way before me, but I just discovered it... so, Im sharing it. :D

Also for those that like to walk around but hate holding down the key, here is the command for making a toggle:

/bind * ++walk