First choice of Fixed Regulator using family IC-78xx. It is easy to use, cheap, popular for a long time.
More about 9 volts DC power supply or 9V voltage regulator
Modify power supply source in old circuit PCB. I then build to is 9v regulated power supply circuit , that can give current 1 to 2Amp. Look 3 circuit below
This is Diy digital voltmeter panel meter 0-50V is 0-50V scale. Display with LED 7 segment. In cost 3$ dollar only, builds simple regulator dc power supply
I collect these transformerless power supply circuit, use a capacitor instead, it smaller and lighter than transformer. There are circuits as follows.
This is the 9V Step-Up Voltage Regulator. We used the RC4193 Micropower Switching Regulator is the base, so low current power output. But The advantage of this system is although input the voltage input change from 2.4V to 10V but a constant voltage output 9 Volt.