I'm sorry to hear about the issues you're facing with your Tower 900. While I may not directly assist with ordering parts, I can definitely guide you on how to proceed.
For parts like the power supply bracket and rubber grommets, it is best to contact the manufacturer directly if general online searches are not fruitful. They might have spare parts available or be able to guide you to a retailer that does.
As for the speed of your son's gaming system, it's a good idea to run a natural speed test once you have everything set up. This will help you ascertain if all parts are working well and if the system is optimized for gaming.
I hope this helps and I wish you and your son the best in this exciting endeavor.