You'll need a 1/4" NPT (National Pipe Thread) tap. They're available in just about any hardware store or Home Depot, etc. Don't go more than 2 or 3 threads deep into the existing threads in the manifold, just enough to get the tapered plug started. The plugs will self-tighten as you screw them in.
A professional-level auto parts store like NAPA, etc. should have the Allen-socket pipe plugs in stock. They're the same kind that are used to plug oil galleries during an engine rebuild. The parts store will also have the tapered tap, I'm sure.
Jerry
