Where can I get the little glass screens for the rear camera lens

They're actually pretty easy to find; if you search for your phone model plus the words "camera lens", you'll come up with all the options you need. Here are a couple of examples.

OEM Rear Back Camera Lens Glass Frame For Motorola Moto G Stylus 5G 2021 XT2131

Amazon.com: 2 Sets PHONSUN Replacement Back Camera Lens Glass for Motorola Moto G Stylus (5G) 2021 XT2131 with Pre-Attached Adhesive Tape/Camera Lens Protector Film/Tool Kit/Installation Instruction : Cell Phones & Accessories

You can find many more on eBay, Amazon and AliExpress, among others.

Replacement is fairly straight-forward; you need to remove the old glass, make sure there aren't any chips or shards of glass left in the camera housing, clean off the old adhesive and glue the new lens on. If the lens is just cracked you'll have less chance of leaving splinters inside, so if it's shattered you should probably use compressed air to blow out the opening to make sure nothing is left inside.