MMANiagara.com

You can order customized shorts from title boxing. I imagine if you order a large enough quanity you could get other items custome made also. I think muaythaistuff.com takes custom orders too

It depends what you mean by "customize"

There are probably websites that give you ideas. . .

Books are another good source.

Or alternatively, you could always be original and do it yourself.

I am not sure why you want to customize it though. . .