I need a rear windage site screw, where would I get one? Or what size is it and what is the thread pattern?

Probably American inch pattern thread. A decent hardware store can measure the thread in the hole and tell you.

VESA® Hole Pitch: 200 x 200 hole pattern / M6 thread size / 14mm hole depth

Bolt size is M6 bolt pattern and thread pitch. A 15 to 25 mm long bolt is recommended depending on the thickness of the wall bracket.

I got got this from GPX. If you email them you can get the full TV guide.

It's a 12mm metric stud with a 1.50 thread pitch.

the mounting bring the right screws for every sides

This can present a problem.
You must know what sizes the screws are. Some cameras have more than four kinds of screws on the case. If the screw is too long, then it can touch some component or the circuit board and could cause a short and ruin the camera.
Try to find someone with one like you have and get the kinds and sizes of the screws. I have Nikons, Canons, Panasonic, Sony and others and hardly any of the screws are the same in the different brands, or differ in kind such as wood type thread for plastic, fine thread, course thread and others. Plus, the sizes of the screw and the length will vary. The colors can vary as well as the type of head of the screw.
Another possibility is to get a broken camera from a site such as Ebay. Buy the camera, remove and use the screws you need, then resell the camera as a repair, as is repair camera.

you can buy allen wrench sets at hardware stores and i'm sure you could find the right size

Very, very difficult re-threading an engine block. Not impossible, but very difficult and not usually performed by the DIY. That said take one of your spark plugs down to your nearest hardware store, one that sells loose nuts & bolts. Try screwing the spark plug into nuts of the proper size until you "match" the thread pattern. Do be afraid to ask for assistance from one of the experienced clerks (its free). The matching thread pattern on the nut will tell you the proper thread size. There are at least a couple different thread sizes and patterns. You may or may not be able to get th tool to clean the threads at the same location. My guess is that they will have the smaller sizes, and you may need to find a more specialized store to find the larger size tools.

You may be looking for the philips flat head 1/4 x 28 tpi (threads per inch) threaded bolt that is about 4" long. The tip of the bolt does not have threads, which aids in locating the threaded hole in the top of the microwave:

General Electric's part number for their OTR microwave cabinet mounting screw is WB01X10180 (or AP3205964 or 942461 as listed on some repair parts sites).

A set of three is GE's p/n WB01X10125 (or AP3185225 or 910236 as listed on some repair parts sites).

This flathead screw is used on most Advantium Speedcook microwave ovens such as the SCA2001BSS03 or SCA2000FWW03 or SCA2001KSS03.

Here's a picture of three screws: