Re: Magpul MAID grip
You can cut down an overly long screw fairly easily as long as you have some way to chase/clean-up the threads after you cut the bolt down. I have not done it with an AR pistol grip screw, but heaven knows it isn't that hard to do and it works well.
If not that, then your local Fastenal (or maybe a general hardware store if you are lucky...my local Ace has hex head cap screws in the right size) will have what you need. Tell them you need a 1/4x28TPI screw (as for the length, factory is around 7/8" to 1" long...can't remember which specifically off the top of my head...so you will want to go down to around 3/4" in length would be my guess). Get whatever head to the screw you prefer (whether its slotted, hex, etc.).
Good luck!