Hello there, I've got the same engine/transmission as you.
All of the actual gearsets for any of the 1ZZ/2ZZ transmissions work with each other, so what you'd be looking for is either a 0.725 5th gear (pulled from a manual transmission Corolla, or bought from Toyota directly (about 230 dollars) or monkeywrenchracing (a little more than Toyota I think, but easier to order), or a whole new transmission with the 3.9something final drive ratio. Since you have a Celica and not an MR2 things are easier since the shift linkages don't need modification.
The best place to look is junked Corollas, because there's no way you're going to find an intact 6 speed to pull stuff from. I'm not sure how easy a MT Corolla transmission is to find though. They go for 2000 dollars on the market.
While you're only dropping highway rpm ~10%, it might be worth it since the 1ZZ loses efficiency pretty fast over 3000rpm, and 3500rpm is only about 64mph in these cars (if you have the same tire diameter as me, which I am not 100% sure about). If you get the Corolla final drive as well you'll have 20% ish rpm reduction which will be very significant, but your acceleration will suffer a bit especially since these are 5 speeds not 6, and the gear ratios have a rather wide spread in 1-4.
By the way, as a fellow early 1ZZ owner, does your engine burn oil? I still haven't driven this 1000 miles yet, but I did get an oil change and the oil level has been dropping slowly but surely (thankfully well under the 1qt/1000 miles that some people see, it's dropped about 1.5mm on the dipstick after 600 miles). Bit of a problem because I'd like to use synthetic oil to lower cold start viscosity, but I don't want it burning off and wasting it. With the high rpms these engines turn thanks to the transmission I feel like oil plays a big part in fuel economy.
Last edited by serialk11r; 11-02-2012 at 06:44 AM..