Re: Custom 5?
You can get Alnico 2, 3, 4, and 5 magnets (and probably ceramics too) from 
www.allstarmagnetics.com.  The magnets are pretty cheap from them ($2.00 a piece) but you have to give a minimum order of $25, which means you have to buy 13 magnets ($26.00).  The upside of that is you can get a few of all four Alnico types to experiment to your heart's content.
Swapping magnets isn't difficult if you have some idea what you're doing.  You have to be very gentle so you don't force anything and tear a wire loose -- or worse -- ram a screwdriver into the coil and kill the pickup.
But basically you 
1)  Remove the black wide surround tape so you can see the magnet to pull it out.
2)  Loosen the four brass screws on the bottom of the baseplate so they stick out about 1/8".
3)  GENTLY pry the coils up from the baseplate so the magnet will be loose enough to pull it out.  You will have more trouble prying up the coil with the adjustable screws because they stick down through the base plate.  Resist the temptation to use a lot of force -- keep in mind that you don't have to remove the coils entirely, just lift them up just enough to be able to slide the stock magnet out and slide the new magnet in, which is only about a millimeter or two.
The magnet is a rectangular metal bar about the same length of the coils (sometimes slightly shorter) that is under the coils and between the two sets of polepieces.  You should see the ends of the magnet protruding from each side of the pickup when you remove the surround tape.
4)  Take your screwdriver (preferable a small one) and place the head against the side of the magnet where all the leads are and firmly but gently push the magnet out.  (If you push it from the other end, you'll be pushing the magnet into the wire leads and you could break them.)
If the pickup is "potted", i.e., it was dipped in hot wax to reduce feedback problems, you will meet more resistance because the wax will have seeped in around the magnet and will act sort of like glue.  BUT AGAIN....resist the temptation to use a lot force or you will be sorry.  If you're patient, it will eventually loosen up and slide out.  Once enough of it is protruding from the pickup to grab it with a pair of needlenose pliers, do so and pull it the rest of the way out.
5)  Take the new magnet and determine which side is north and south.  The poles are on the sides of the magnet, not the ends.  The north side should face the slug coils, the south side should face the screw coils.  If you put the magnet in the wrong way, the pickup will be out of phase with the other pickup(s) in the guitar and you will get a hollow, nasal tone when you use the pickup together with the other pickups that you probably won't like (though some do).  
It's best to use a polarity tester to determine the polarity of each side of the magnet -- but if you do put it in the wrong way, all you have to do is repeat the above process and flip the magnet over (side over side, NOT end over end).
6) Push the new magnet all the way into position and then re-tighten the four brass screws on the baseplate and you're done.