My league has 5 prospects spots per team (100 team league). Those guys are taken in a supplemental farm draft before our auction every spring. I make a list of every top 80 or so I can get my hands on. Most of the time many of the guys are already taken from previous years, but I will rank 50-60 guys. I use every list I can get access too, some free, some paid. Last year I used, Keith Law, Kiley McDaniel, Fantrax, TJ Stats on twitter, and fantasy pros. I think Rotowire and Rotoballer also have them, as well as BA, BP, and Fangraghs. I make a composite list and then read through as many profiles as I have time for. I'm looking for things that are more fantasy relevant. I also prefer hitters over pitchers and I like upside more than floor. In our league, you can draft anyone so I look at early draft boards and see if there is a top 30 prospect that is about to get drafted.