Where to find cheap 5.5x2.1mm PCB-mount barrel jack connectors?

As with most commodity PCB components, LCSC is a good place to find cheap connectors if you want to buy more than 10 pcs or so (due to the shipping fees).

These are listed in AC/DC power connectors. In order to find them quickly, select the Package=Plugin filter and select In Stock and then sort by price (ascending).

Using this method, you can easily find parts such as:

Keep in mind that these are not 100% the same regarding their geometry, so always check in the datasheet if they fit your footprint.