Racepres: I have always had the best of luck calling Winchester/Browning for the parts I've needed. Their parts people are very nice and have always helped me out.
The mag tube cap plug screw shouldn't be a problem. If there is a question as to the right one, they will probably send a couple just to be sure.
Regarding the mag tube itself - are you planning on cutting it down? Is it a full length tube now? Why would you want to shorten it - is it buggered up - if so, just ask Winchester for a new one - they have done that for me without a problem and they are pretty easy to install. HTH. Mikey.