There are some universal remotes you can buy that can operate OEM keyless systems but they aren't very good IMO. Besides, you need a working remote to copy from. It's quite easy to get OEM remotes, I see them on e-bay once in a while. There is a web site I've used occationally for hard to find remotes.
http://www.remotesunlimited.comPart number 101-1101 they want $144.95 for it.
Joe @ Infiniti of Scottsdale has them for $94.81. I notice you are here in Phoenix. If you want that price you need to call him first. He's not supposed to sell at his discounted prices at the counter.
I only have one but never use it as I have an aftermarket system but I don't think I want to part with it in case I need it for some reason some day.
For $100 (the cost of one remote) you can get a pretty nice aftermarket keyless system. The locks and trunk are simple, even the fuel door, and with a little research and testing the window roll down with the OEM system should be duplicatable. Last resort you could add your own window roll up/down modules but using the OEM system would be cool.
I'm in the Phoenix area so if you want any help let me know.