I bought the Galaxy Tab A 8.0 on a plan with no s pen, if I purchased an s pen separately, would it work on the Tab A 8.0? If not, is there anything similar I can buy that resembles a pen for more precision?
The s pen appears to be compatible with your device. But make sure that you purchase an official Samsung s pen or the after market pen my not function correctly with your tablet.
Before actually purchasing a S Pen, try borrowing one from someone you know, or visit a store that sells devices that use a S Pen. All S Pens will work on any Samsung device that has the Wacom screen layer, so you should be able to test your tablet quite quickly if its a go or no-go.