I really don't have much in terms of "F" artists, so I'm going to go with the one whom I've had the most contact with: Dave Fox.
I usually see Dave at New York-based conventions, particularly at the Big Apple Convention early in the year. His rates are reasonable, but I'm either enticed by other artists or I'm running out of money. He's done good work with the references that I've given him. Two of the sketches were on blank-covered comics, which is a bit of a drug for me. I wind up buying them at shows and shops, but I don't get around to having them sketch on. Here is Dave's take on Chris Hemsworth as Thor on The Mighty Thor #1 . . .
. . . and here is the obligatory "Spider-Gwen" commission that I figure every collector has to obtain these days.
Dave also did a kick-ass take on Confessor, the Batman expy from Astro City.
If you see Dave at a con, hit him up for a commission. You can see more samples of his work here and here.
HONORABLE MENTIONS: Bob Fingerman (one sketch from 2009), Chris Flick (one sketch that kicked off my series of "South Park characters as Lanterns" commissions; got posted in ComicsAlliance's "Best Art Ever (This Week)"), Brendon & Brian Fraim (two sketches from these brothers; the only times I've gotten commissions from two artists), Simon Fraser (two Doctor Who-themed sketches, including one from Long Island Who last year).