They're not that common but they're easier to get than you might think. If Lews Guitars doesn't work out, Lindy Fralin will sell you some (that's where I got mine) as will www.allstarmagnetics.com.
Fralin's are pricier ($4-5 each) but you can buy as few as you want. Allstar Magnetics (where I got my Alnico 3's) sells 'em cheap ($2 each) but the minimum order is now $50, so you'd have to buy 25. But that's not so bad because you could get a half dozen of each variety, Alnico 2, 3, 4, and 5, and possibly Alnico 8 if you're interested in experimenting further.