The 4-color round Simon game tests how good is your memory. You can play it for free in the browser as of this writing here: https://freesimon.org/
If you want to buy a real Simon, pretty much any store that sells games has you covered, Walmart, Amazon, you name it. Will set you back around $20 these days for the fullsize original Hasbro toy game Simon.
No links on this one, you can find it by just typing “simon” into your favorite online store or, hey, here’s one people don’t do as often: get in the car and go down the toy games aisle, usually where the board games are at Simon can be found.
If you don’t care about the brand Simon, you can save some $$, then there are plenty of knockoffs that have basically the same game, but in different shape and size. Pictured below is one from tingbowie (huh, what?) and just has a generic name and will set you back around $9 USD.
For more games you can play linked from PlayGamesMore, just visit the tag play.