I'm definitely going to agree with GLXLR on this one... I'd rather not have people a) know I'm carrying around an expensive bit of kit, regardless of the brand, b) have some snob come up and criticize my brand choice (this has actually happened to me... some Nikon/Canon owners just don't know how nice Pentax/Oly/Sony equipment can be), or c) have a massively uncomfortable strap with PENTAX embroidered all over it digging into my neck all day. I ditched the provided strap as soon as I could after getting my K200D, and replaced it with a large, black, unmarked, padded LowePro strap... one of the best purchases I've made for the camera, I think, because I can wear it all day without problems.
That being said, I just did a Google search and couldn't really find anything in the way of Pentax straps outside of replacements... the typical red emroidered ones or those with the embossed leather that come packaged with their cameras/binocts/etc.
I suppose if nothing else, you could make one yourself!
Nikon D300 / 35mm f1.8 / 300mm f4 / TC-14E II
Pentax K200D / DA 18-55mm / DA 55-300mm