Where the best place to get an "ignition on" signal for my fuel pressure gauge and future add-ons?
Please help!

 

There are plenty, but here is one in particular (see post#3): https://ls1tech.com/forums/showthread.php?t=265663

 

...because some of the signals are not a full 12.0 voltage and some are on the timer like when you shut off the truck the windows and radio and such will still work for a while until you open the door or remove the key.
Get a meter and make sure the one you select has at least 12.0 volts.
I used a test light to pick one for my Viair 450c compressor. Being controlled by a solenoid, some times it would work and sometimes not. I did lots of painful trouble shooting before I got a meter and found the pin I selected was only 9 volts.