I don't know where you can find replacement screws. I don't use screws for my case covers. I use stainless 6mm socket head bolts. You can buy "kits" through a company like
www.z1enterprises.com for many bike models. If a "kit" isn't available, you can remove and measure your phillips screws for length (they are 6mm with 1.0 pitch) and order them from Maryland Metrics in stainless/socket head. Don't buy zinc or plain steel due to rust.
You ask if you should use an impact driver on the side cover. Do you mean the screws that hold the side cover in place? If so, the answer is yes. A hand impact driver with the CORRECT phillips bit is the ONLY way you can get most cover fasteners off that haven't been off for a long time. If you mean should you hit the side cover itself with the impact driver, I doubt that will do much good.