It's been a few years since I had to, but when I replaced a couple on my 23.5, I got the replacements from Sailrite.
http://www.sailrite.com/easearch#!&ea_q=sail slug
I also got some webbing, large needles, and waxed thread from them. I made my own sewing palm. You can also order one from them. After that, it's just stitch picking the old webbing stitches out, so you can swap the slugs, and hand stitching the new one in place without putting the butt end of the needle through your hand too many times.
Of course, you may just find it easier to just send the sail out for repair, but it's a useful skill to learn to do yourself.
edit:
Just found a copy of my sailrite order in my emails. I ordered both #23307 slugs and a couple of 23312 slugs. I think I used the 23307 slugs. I also ordered #24105 screw on plastic shackles, but II remember them being a size too small. I also ordered #29206 waxed twine and a supply of #15, #16, and #17 hand sewing needles. The larger of the 3 sizes are probably the ones you will end up using, though you want to use the smallest you can get away with using.
