You can buy Gates replacement hoses at most parts stores, which is the route that I went. I replaced my upper heater hoses proactively when I replaced my starter, all of them were actually in great shape inside and out (maybe they had been replaced before I bought the GX). Gates hoses are around $10-20 each. I actually didn't replace any of the lower hoses heater on the engine, but I did replace the radiator hoses.
If the hose has any sharp bends on it, I would get a molded hose (Gates if they make it, OEM Toyota if not). If it does not have any sharp bends there is no reason why you can't use generic heater hose (probably 5/8") from the auto parts store. The generic heater hose will kink like a garden hose over a sharp bend, but works fine for smooth bends. I've actually used it for sharp bends before, but put a nice big loop in it so as not to kink it (obviously it's not feasible to do that everywhere).
I would also replace the thermostat and radiator cap with genuine Lexus/Toyota parts while you are in there, the upper and lower radiator hoses, and give the cooling system a full flush.