just go to lowes or home depot, go to the plumbing fittings, and find the compression cap that looks like it will fit. IIRC its a brass 5/8 compression cap.
lol. i actually cut the tube and took off the "screw" that holds the tube to the header. and i put 2 pennies in it with lots of exhaust gasket making stuff, layered that all over. and it made a nice looking cap and i havent had any problems
Or... why dont you just bring the EGR tube into the hardware store and find the adapter that threads into it... and then buy a cap for that thread size. Done.