What is the best way to get concrete?

The most reliable, renewable (yet least cost effective, cap-wise) source of concrete are concrete shipments, which can be purchased from a few locations:

• Connie Abernathy (Abernathy Farm)
• Supervisor Greene (Greygarden)
• Daniel Finch (Finch Farm)

If you bring the shipment to the workbench at the desired settlement, it'll deposit the material into that settlement's inventory.

Bags of Cement can also be found throughout the Commonwealth, and are often found in bulk at more industrial locations, such as factories and shipyards. A list of some good places to grab some can be found in this Fallout Wikia article.

Past that, you can scrap destroyed houses or individual/walls of cinderblocks at various settlements. Hangman's Alley and Greygarden stand out to me as places that had a large number of salvageable cinderblock stacks. With the concrete scrapped, you can either manually move it by taking it from a settlement workshop and depositing it into Starlight Drive-In workbench or by Setting up Supply Lines to share the resource from another settlement.