Pickup Wizards hot-wound Jazzmaster pickup is my recommendation.
Probably want a pickup in Alnico 2 magnet territory, the lower the number, the warmer the tone from my experience (Alnico 5's are a bit sharper sounding).
The Jazzmaster I built back in 2010 has a Pickup Wizards Alnico 2 Jazzmaster pickup wound slightly hotter than normal (8.8K, vs. the usual 7.5-8K most Jazzmaster pickups are wound to), a lower number Alnico magnet and a higher number of winds will get a warmer pickup. Lollar, Novak, and PIckup Wizards can do that.
