How do I find a replacement rear axle?
Your bike appears to have "Giant Tracker Sport Disc" hubs. They appear to use cartridge bearings. Additionally, some service manual PDFs found online make it appear axle removal is a somewhat involved process, and so your axle removal may have disturbed some bearings. It also appears to use a proprietary axle type but I'm not 100% on that.

Take your bike to where you bought it or another giant dealer, as they'll have the specialized tooling needed to fix your bearings and will be able to order a new axle.