You can find cheap adaptors with multiple barrel jack sizes, for example: https://www.amazon.com/Belker-Universal-Connector-Frequently-Interface/dp/B09DCL56XM/ref=asc_df_B09DCL56XM/

That symbol shows the polarity of the plug. In your case, it's "center positive" (as opposed to the other option which is "center negative")

You haven't provided a picture showing the actual port where the power cable plugs in, but it's almost certainly a 5.5mm barrel connector, with either a 2.1mm or 2.5mm diameter center hole. (#5 or #6 in this diagram).

It's also worth noting that the current requirement of the antenna is listed on the label in your 2nd image (50 milliamp). Normally you'd want to make sure whatever power block you buy can supply enough current but, in your case, 50ma is low enough it won't be a concern. In fact, I believe you could power this antenna from a USB port if there happened to be one in the vicinity (with a cable like this.)

Either of the adapters /u/mtinez links to should work. Just be aware that you need to set the proper voltage first(!), and configure it for the right polarity (again, "center positive").

Before you do that, though, you might hunt around your house to see if you have a compatible power adapter floating around. These kinds of adapters are super common. Have a look at any old rechargeable tools you might have (drill, screwdriver), kids toys, or commodity computer networking equipment, handheld vacuums, cordless air pumps.... whatever. Most adapters will show both the voltage and the plug polarity (that symbol you mentioned). As long as those two things are the same and the plug fits, the adapter should work.