Below is a complete list for the locations of all minikits in Chapter 15: Ultron Undone.
Part A MInikits
Minikit #01: This one is found surrounded by a ring of studs in the sky.
Minikit #02: You must destroy a total of three gold ships flying over the helicarrier.
Minikit #03: Keep an eye out for a cage tied to some balloons. Smash it.
Part B MInikits
Minikit #04: During the battle with Ultron, send someone to fly up to the archway just to the right of the cracked wall.
Minikit #05: Search for an alcove just to the left of the cracked wall, and have a brawny character pull at the green handles, to find some pieces you can reassemble into a minikit.
Minikit #06: Search for an alcove just to the right of the cracked golden wall, and have Ultron do his thing on the pile of debris there.
Minikit #07: Check for a dig spot next to the alcove to the right of the cracked wall. This collectible is buried there.
Part C MInikits
Minikit #08: Check the section next to the green vehicle to find a red handle on it for Hawkeye to use.
Minikit #09: Finding this piece requires a bit more effort, with you having to dig up boxes buried all over the area. There's one in the west alcove of area 1; another to the far left of area 2 and the last one is underneath a crate in an alcove in area 3.
Minikit #10: This one is buried under the dig spot in an alcove off to the right side of area 2.