You can get ideas and slugs at Sailrite:
http://www.sailrite.com/
You'd probably have to sew in backing patches for the slugs.
If your sails are very old you might consider just getting a new mainsail. After estimating the costs of materials (not much) and labor (how much is your time worth?) in installing your own slugs, the cost of a new mainsail might not look so bad.
But a first option to try would be to really clean out the bolt rope slot in the mast and lubricate it with Sailkote. Get a section of cheap line that just fits inside the slot and use something like simple green to "scrub" the slot, then rinse and dry and lubricate.
A sailmaker showed me a neat trick - using a butane fireplace lighter (bigger than a cigarette lighter) pass a quick flame over the boltrope. This will singe off the fuzzies on old boltropes and make it smoother and tighter and harder. Then use beeswax or paraffin to wax up the boltrope. This will make it easier to get in and out of the slot and slide.
Carlos