Welcome to the forum!
A money-saving tip for buying exhaust manifolds... Buy an IS exhaust manifold! The 98-05 GS300 and 01-05 IS300 share the same exhaust manifold from the factory. Aftermarket IS300 headers are around $150, and I have the "OBX-style" headers on my GS300. I bought mine on eBay. If I were you, I'd just find a shop through google if I could.
The header I bought had the lovely Chinese welding teqnique... MIG weld everything, grind down the welds and TIG over them so they looks nice, and then polish the piece up. It doesn't look top or anything but does the job.
I've run into 0 problems so far. No leaks, no heat issues, and no modification needed to fit the headers.
A word of caution, though. If you will be installing them yourself, have at least one buddy help you out. Oh, and there is an exhaust mount/hanger of sorts that connects to the transmission housing that might cause issues while you remove and install headers. Remove the mount and it makes things a little easier. Consider buying new OEM (or better) exhaust gaskets. The ones that may come with headers you purchase might be of questionable functional value.