Welcome @mkmck!
Amazon has 1,2,3 and 4 camera starter bundles including a base. They also sell add-on cameras.
https://www.amazon.com/Outdoor-Wire-Free-Assistant-station-required/dp/B08FFK9ZYR/ref=asc_df_B08FFK9ZYR/?tag=hyprod-20&linkCode=df0&hvadid=475739861994&hvpos=&hvnetw=g&hvrand=2847866299821805720&hvpone=&hvptwo=&hvqmt=&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=9001677&hvtargid=pla-1016379457166&psc=1
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