You'll have more luck with the second tier top prospects. Kyle Zimmer, Taijuan Walker (who could be considered first tier), Jose Berrios, Richie Shaffer, and Bruce Maxwell all come to mind.
Austin Meadows is also a great signer. Clint Frazier signed before he got drafted, but I'm unsure about how he goes about his fan mail now.
Autographed 2013 Topps Pro Debut Progess
ISO: Daniel Robertson 2014 Bowman Platinum Canary Refractor Autograph /15