tireminder.com looks promising, although I couldn't find pricing for their motorcycle system. They appear to be big in motorhome and trailer tire monitoring. 2 larger caps that screw on your stem that transmit tire pressure to a phone app. You can get alerts if they fall below a certain threshold.
Amazon is full of cheapies that are just caps preset to between 32-40 psi. They will go from green to yellow to red as pressure drops.
I was laughing at myself the other day...checking tire pressure on Nighthawk, 32 psi is target, both were at 31.5. It took everything I had to not grab the pump and add the additional .5 lb of air! (I did the next time I rode as they were both down a full 1 pound, unacceptable!)