Epson 4900 ICC Profiles
I am not surprised you can't find profiles for those paper, my advice is if your going to continue to use them either find a paper with a profile that is a good match for the COSTCO paper, I am pretty sure Costco does not make their own paper. For example, there is a paper line called PROOFLINE that makes a luster and enhanced mat paper that are very close matches to Epson luster and mat, and I have been told the same company makes them.
The paper is very cost effective and offers good value for the money and has good profile matches with an epson 4900 (which is what i use)
The more professional way to do this would be to purchase a Color Munki and make you own profiles, but only you can weigh the benefit of purchasing this device in order to print on third party papers. I have one and have done some tests with comparing the factory profiles with the ones I made with the Color munki and they are very, very close.