As LegitShady said, Amazon, or most other electronics sellers (eBuyer, etc.). The one I have is 14" 1080P (matching my laptop's main screen very closely) from “ARZOPA” but there are several similar on the market (I suspect at least one is exactly the same with a different label…). As you can see from
https://uk.camelcamelcamel.com/product/B09TVVWFR9 the prices of these things bounce around almost weekly so shop carefully. There are sometimes “vouchers” that don't show up on CamelCamelCamel, I got an extra tenner off at a point when the screen was at ~£90.
There are also a variety of different attachment options, from no attaching to your machine (which I find preferable) to those that attach to your laptop and add two screens, one either side.
Watch out of fly-by-night kickstarter projects in this area, there have been a few in recent years including a monster that promised to add three monitors to your laptop (left, right, top) which I'm surprised people were taken in by (I found the extra weight of one less than convenient, even when sat at a desk, never mind three, hence currently using a stand-alone external screen).