For RCX bricks with a power port, the LEGO recommendation was their 9V AC Adapter part # 9833.
FTDI chips generally have the best reputation, so more general advice is to look for adapters with those chips.
- USB-to-Serial Adapter Cable by ESU - ESU needed reliable adapters to work with the serial connections on their hardware, so they have a vested interest in ensuring that these adapters are functionally robust.
Due to the design of the serial IR tower, a "dummy" 9V battery is the easiest replacement. No fees, commissions, etc. are earned with these links; other sources welcome.
- 9V Active Dummy Cell Battery with 8” Flat Flexible Cable and 5.5mm/2.1mm Barrel Socket - For use with a separate power adapter
- USB-A to to 2.1mm Jack with 9V Output
- USB-C Power Delivery Adapter with 9V 5A Output - The jack works with sockets having a 5.5mm outer diameter and either a 2.5mm or 2.1mm inner-diameter
- USB-A to 5.5mm/2.1mm Jack DC Booster Cable with Selectable 9V or 12V Output
- USB-C Power Delivery Adapter with Reprogrammable Output Voltage - The jack works with sockets having a 5.5mm outer diameter and either a 2.5mm or 2.1mm inner-diameter
- 9V Active Dummy Cell Battery with 8” Flat Flexible Cable and Power Adapter
- Alternative source for 9V battery eliminators (much more expensive)