I've been collecting license plates for a bike trailer project since 2016. I started by finding many on the road side. I bought some at general swap meets.
I put a plates wanted ad on craigslist one time that lead to the discovery of the ALPCA: Automobile License Plate Collectors Association
The Automobile License Plate Collectors Association - the largest and most respected club for license plate collectors and aficionados all over the world.
www.alpca.org
They have many official and independent swap meets listed on their website. There may be one near you. I went to an unofficial swap meet and it was pretty interesting. They had specialty displays with judging. And they sell run of the mill plates usually for $1 or $2. For collectors, there are far to many to collect so specializing is a must. Like all tribal plates or one guy I met is trying to find all counties in his home state from the year he was born. All 1976 plates (bi-centennial designs) is a possibility. One swap meet display was all demo plates from a courthouse where all the numbers were zeros covering many decades.
There are countless plates on ebay including the 50-state sets.
I put every valuable plate I get on ebay. The most I ever got for one was $150. I'm not looking for expensive, just run of the mill plates I can bend, spindle and mutilate. I made a yard spinner with a bicycle wheel and plates as fan blades to catch the wind. I was thinking of a Rhodes 4 wheel pedal car with a full license plate body but where would I store it. I've seen many plates used for art projects like bird house roofs or cut out all the letters to respell other words.
I called my county office about getting old plates. You would think I was trying to rob fort knox. They told me all the turned in plates get shredded under deputy guard and the metal shreds sent to recycle.