Where can I find Halloween costumes in a 6X?

You might have to get a little creative with these! I can find up to 4x in these sites but not 6x. They might have to be specially ordered in that size. Or I suggest checking out some of the sites & working with your own wardrobe and adding accessories to them to create a costume.
For example the Hellraiser comes with a mask, robe & weapons, you could supply the rest! ; )
Hellraiser-Butterball Deluxe Plus Size Adult Halloween Costume
   

This Officially Licensed Big and Tall Adult Halloween Costume Includes:
Mask, Robe, and Weapons

Does not include pants and shoes.

100% Polyester, See package for care instructions, Imported

Sizes vary by costume. Be sure to check your measurements below.

If you are in doubt between two sizes, it’s usually better to order the larger size for a more comfortable fit.