You can buy them on Amazon and I presume other places, but I don’t think it’s a significant enough problem to worry about. Thieves are just car jacking the entire vehicle these days.
Bell 22-1-45915-8 Anti-Theft License Plate Fastener by Amazon.com Learn more: https://www.amazon.com/dp/B000SOM9GQ/ref=cm_sw_em_r_mt_dp_EGJ5S1QX98M9492F65NR
Another tip if you get personalized plates make sure you get some sort of anti-theft device for your plates (both front and back). The reason is if one or both of your plates are stolen the same plate cannot be reissued so you would "lose" your personalized plate.
Here are a couple of examples of anti-theft devices.