rear glass replacement I don't see were to buy

Generally if you search for something along the lines of "iPhone XR rear glass replacement", you'll come up with several sources to buy a piece of glass. Here's the first example that came up when I put in that search.

Amazon.com: Ankeca Original Back Glass Replacement for iPhone XR 6.1 Inches All Carriers with Pre-Installed Adhesive and Repair Tool Kits (White) : Cell Phones & Accessories

Of course you'll want to pick the correct one for the color of your phone, or you can opt to select a different color if you feel like a change. Note that there are two different versions of glass you can buy; one of them, like the ones being sold here on iFixit, has the camera bezel holes cut out at the original size, while you can buy others with the camera holes enlarged, sometimes called a "big hole" version.

The difference is that the hole is made bigger so it will fit over the camera bezel. When the phone is manufactured, the glass is put on before the bezel is installed and spot welded. The bezel overlaps the glass so if you use one with the original size hole, you must detach the bezel first before installing the glass. It's not easy, as you either have to chisel it off or possibly grind it off with a dremel tool, then you have to figure out how to reattach it since you probably don't have the micro spot welder Apple uses. The big hole version is designed to fit over the bezel and avoid that whole problem, but the trade-off is that there's more room for dust, dirt, or water to get in with a glass that doesn't fit as tightly to the bezel. My usual suggestion is to seal the bezel with some silicone sealant when you're assembling it; that should help to keep liquids out.

This is a difficult repair and one that is easy to mess up and cause damage to the phone due to the prying and heat that's involved in the process, so you'll want to work carefully and methodically and follow whatever YouTube video you've decided to use as your guide.

The other alternative would be to purchase a new or used complete rear housing with the glass already installed. If you did that, you would then essentially gut the phone and transfer all of the internals to the new housing. That presents less risk of heat or prying damage, but has its own set of hazards as it's still possible to damage the components while removing and replacing them.

When I've had to do the same sort of thing on an iPhone X, I've so far opted for the housing replacement strategy, but have heard from many people who've done successful glass replacements too, so you'll have to judge for yourself which one you want to tackle.

Either way, let us know how it all works out for you, and best of luck with your repair.