I just got my first telescope, a Celestron C8-XLT on AVX mount. It has the black metric philips screws. I wanted to replace them before my first collimation attempt (wish me luck). I have a GoldFocus mask I plan to use for collimation which makes it hard to use thumbscrews, and the makers of the mask recommend a T-handle hex driver for more torque and finer control.
I bought a pack of 3 hex socket cap screws at Home Depot for $0.45 and they fit nicely. They are M3 with 0.5 threads and have a 2.5mm hex socket. I got 10mm long screws because they didn't have them in 12mm (which other posters have used). Regarding the length, even at 10mm these are longer than the original philips screws. I've attached a picture of original screw next to with the ones from Home Depot and a picture of the package.
To adjust them, I've ordered a Klein Tools JTH6M25BE 2.5mm Hex Key with Ball-End Journeyman T-Handle ($4.22 on Amazon). Until it arrives, I'm using a regular allen wrench (L-shaped). While traditional allen keys are clumsy, the T-handle looks ergonomic and should give me fine control.