[Willard]

Anybody know where I can find a flared magwell and extended base plates for my MK25?

As already mentioned, Hogue makes what they call Magrips… https://www.hogueinc.com/grips/sigsauer/p226/dasa/g10/magrip

or Sig's TACOPs grips... https://www.midwestgunworks.com/page/mgwi/prod/GRIP-226-BLKPOLY-MW

Mec-Gar 20 round magazines... Sig P226 20 RD 9mm anti-friction coating Mec-Gar MGP22620AFC [MGP22620AFC] - $29.95 : Greg Cote, LLC

Extensions for your current magazines will depend on their manufacturer...