Originally Posted by
WhyFi
Don't change your saddle position to address reach; saddle adjustments are for your position relative to the pedals/crankset.
While 90mm is a very common stem length, many Giants have oversized steerers, so you may have a limited selection. Have you contacted the shop where you got the bike? They should have bins of compatible stems around for you to try or buy.
Giant has a steering tube/stem adapter shim. It usually comes with one of their stems purchased separate from a bike. I have one laying around. Somewhere. But I agree with this poster above.