Re: looking for action screw for a rem 700
If it's just the screw that's stripped, & the action's fine, first try calling/visiting any & every place local that does any smithing/sells firearms. Look at it as a chance to get to know a few smiths in your area, particularly ones you didn't know existed before.
I know for bedding I just use 1/4x28 bolts with the heads cut off. Though they're a bit loose thread fit wise were I thinking on bolting an action in place with'em. Time for me to buy a thread pitch gauge, well, past time, but...
That said, likely some other Hide members will chime in.