I use Tegaderm usually does the job water proof stays on OK never had a sensor dislodged while using it just have to be careful to get the correct size for what you want. I get it on Amazon.
Edit to say sorry forgot to say hi and wecome to the foruns.
I use nothing. I find they stick well to me and do not need to add more single use waste to the landfill.
Some people have purchased 3D printed straps from eBay. These are reused.
For me, Hypafix or Tegaderm as well.
As for where to get it, google it and order from the cheapest seller.
Or ask your pharmacist if you don't want to order online.