There are three workable ways that I have seen modular tanks accomplished, two of them are presented on the 40K.ca Message Board by She and Ski and Zustiur. The following is my own critique of the methods they present, my own Method can be found on my Vehicle Page
She and Ski's method is elegant and relatively inexpensive but I've veered away from his approach for a few reasons:
Zustiur approach is more fraught with difficulty, at least when I tried to implement it
Elsewhere on the net I have seen some other variations on Zustiurs approach, so there are at least two people on the planet with access to smaller magnets than I (grrrr). Anywho if you find very small rare earth magnets available anywhere drop me an e-mail
Update: A recent visitor to my site was kind enough to submit an online store Gaussboys online store which sells a variety of nifty magnets, including miniscule 1/16" by 1/8" diameter magnets. These which would be handy if you wanted to do Zustiur's approach. (Thanks for the link Matt)