I've recently been buying up several guns, including AR's and I've gotten to the point where I know enough to know I don't know enough. I've been trying out different pistol grips, and I've run out of washers. I know I don't have to use them all the time, but I prefer to do things properly. I was wondering if I could buy a cheap box on amazon with free next day shipping, instead of ordering from brownells or optics planet and having to pay twice the price in shippling and waiting a few days for it to arrive. Does anyone know what size washer I would need. It would be great if I could just buy 100 of them and not have to worry it anymore. thanks for the help.

The right thing to use is a 1/4" external tooth Keps lockwasher - they should be available at almost any hardware store for the cheap.

would this work?

https://www.amazon.com/gp/product/B071J75LLQ/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1

Yes, those would work fine.

Thanks, much appreciated.

just wondering, if you're using the socket-head cap screw instead of the slotted screw, would an internal-tooth lock-washer be more appropriate?

Yes.  The socket head is too narrow and will sit inside the teeth without touching them.