I like the Scale Motorsport ones a lot. But, while one package is two harnasses, they are two different harnasses, so if you are doing a tuner, the two seats won't match unless you get two packages and get the two like harnasses out of each. But you can do two cars, then, so it's not so bad.
Also, I just got the NSX PE set from Tamiya, and it includes a really great material for seatbelts (I think?). Anyone know where I can get more of this colored vinyl material?