This post talks about this subject a little bit:

quality of pickup magnets - Seymour Duncan User Group Forums


I think that while it may make some difference, it's probably not as much of a difference as the type (alnico vs ceramic) or strength (2, 3, 4, 5, 8), or gauge wire you use and how many windings you use on the pickup, if you make them "scatter wound", the pole pieces and how they work within the package, etc...

However, maybe you can send Lindy Fralin or Jason Lollar an e-mail and see what they say. My guess is that where possible they probably use NOS U.S. magnets/magnet stock.