Edited from my earlier reply in December (#10) to reflect better hafting methods...
If you cain't get warranty handles, make a note of the lengths you want and go to as close to an old-timey hdwr store you can find, like an Ace or a True Value that has their stuff piled one on top of the other. They're more likely to have replacement handles than a too organized big box store. Bring the heads with you so you can match the size and tongues correctly. I would clean the heads leaving as much original finish as possible and wax them rather than restore to like new.
I like to find handles with the tongue just a little thicker than the hole. As you're test fitting, make sure the head goes on the handle the same way each time. Start by shaving down the tongue but with a taper at the shoulder til it fits into the head but say 1/4" short. You should see the taper coming out the bottom of the head. You'll want the tongue extending out of the head on top by 1/4" or more when done. [Don't bother with my earlier suggestion on scribing a line and cutting a step into the shoulder. It's not necessary.] As far as how tight to make the tongue at this point, you'll have to consider the wedges you're going to use and size the tongue accordingly. Always sneak up to the right size. You'll want the wedgeless tongue to fit "fairly" snug to the head at the shoulder before it's finally seated, again though, depending on the wedges.
Strip/sand off the factory finish and put your own on. I use a little bit of stain and then a couple of coats of boiled linseed oil. Dunk the tongue into an open container of BLO along with the wedge for a day or so. Assemble by putting the head on as far as you can by hand, hold it upside down, and firmly pound the end of the handle with a mallet which drives the handle tightly into the head. Put the wooden wedge in but don't hammer it in all the way. Let the handle dry/adjust to the BLO for maybe a week then finish driving in the wedge. Cut the protruding tongue so that it extends maybe 1/8+" on top and add a metal wedge if you use one.
Fill a metal jar lid to nearly the top with BLO and situate the hammer upside down so that the protruding tongue sits in the oil. Let it soak for several days, making sure to drip a bunch of oil into the eye from the shoulder as often as you think about it. Let dry. Fondle proudly. Use. Add more BLO to the handle a few days later then every so often. Be sure to specify who it goes to in your will.