The Roll 7 Devolopers heads are spread thoughout 10 of the levels in Career mode. The heads are found in a mix of Amateur and Pro levels. I suggest having most or all of the levels unlocked before you try to collect them all.
This is the Profile screen. The second to bottom entry tells you how many Developers heads you've found. Below is a video for the PS4 that covers all 10 in less than 10 minutes.https://m.youtube.com/watch?v=yl2epYMuC-w
Credit to: SBDgamingHD