How can I find all the air leaks and insulation gaps in my finished attic?
Two separate issues here:
Attics Get Hot!
This is a fact of life. Attics heat up from the sun all day long. Color and type of roof can make some difference (reflection vs. absorption), but it is quite normal for an attic to get really hot. The usual solution is an attic fan. This can be thermostatically controlled so it only runs when hot. For an attic fan to work, you need to have the fan on one end and air coming in from elsewhere (but not from your house below).
If your attic is really getting hot from your house, or if your attic gets hot and the hot air goes down into your house, then you get to checking for leaks:
Thermal Imaging
There are thermal (infrared) cameras available as standalone devices, and also as add-ons for handheld computers (aka smart phones):

You can either buy something (if you like gadgets and can justify it to yourself) or borrow or rent one.