I've posted about it before, but I did a 12" barrel on an NEF Pardner (870 clone). Going from memory, but I'm almost certain that the magazine tube is silver soldered into the receiver. That's a good reason not to find a bunch available on the market--it's not an easily removable part by the end user.
So what I did is to strip everything down to just the bare receiver and magazine tube. I cut the threaded end of the magazine tube off, then cut out the section I needed to reduce the length by the amount desired, and then welded the threaded section back on. Some work with a Dremel sanding drum smoothed out the inside of the tube, as well as the outside. Then I cold-blued it. When I shortened the barrel, I had to cut off the lug that joins the barrel to the magazine tube, so when I welded it back on to the barrel, I made sure to do that so it would cover the welded section of the magazine tube when assembled.
With a 12" barrel and a magazine tube that is just barely shorter than the barrel, I get 4 2 3/4 shells in the tube. (I don't keep one chambered on that gun usually, but if I did, 4+1.)
You could easily do a 10" barrel if you're okay with 3 in the tube, but then you get into issues with shortening the forearm. With a 12" barrel, the barrel just barely peeks past the end of a standard-length foreend.
Aaron
Edit: I posted some pictures the last time this topic came up:
http://www.thehighroad.org/showthread.php?t=778836
But I see that you said that you didn't want to mess with cutting the magazine tube. Honestly, unless you're going to have a gunsmith do the work, I don't know that there's any "pay for it" options for shorter magazine tubes. But I could be wrong...