They randomly show up throughout the season. The amount of money in your IFA budget determines how many you'll see, as well as the accuracy of the projections you will see for them.
The stud IFA's who show up will cost you mega-bucks. It's not unusual to see them going for over $20m in bonus money. If you're not willing to spend that much, then the IFA's you'll be willing to afford will tend to not be the impact players.
You'll tend to get much better value for your prospect money from the amateur draft.