Roundabout

To unlock the pickup finder, you need to complete the objective in the power-up shop for finding 50% of the cash pickups.

I don't have a map to post of all the pickups, but here are some recommendations:

-Be on the lookout for them as you complete the story missions or are driving between them. Finding 50% shouldn't be difficult since a lot of them are in the open. You likely won't get the time objectives on your first time through a mission anyway so take them slowly and focus on the collectibles you see

-If you're struggling, take the inflataboat power-up and roam the waters of the 3 maps. There are a bunch of smaller islands that house jump, cash, or horn pickups.

-Some cash pickups are usually just in a tight corner. Don't worry about dying right after you pick it up since it will still count. But if you can't fit, consider completing the campaign and taking the limo shrink power-up with you.

-Travel the rooftops/high terrains. The mall in Area 1 and some of the rooftop areas/cliffs give you a view of where additional cash stacks are.