Finding the diodes and caps is easy but I am struggling g to find a "24v 3A transformer"
Most things I find are AC to DC supplies which I don't want (do I?) I assume I want just a plain AC to AC transformer, but they seem to be hard to find and are not rated for a current as it seems to imply.
I am struggling g to find a "24v 3A transformer"
24V and 3A is a maximum VA (volts times amps) of 72 VA (some would also call this 72 watts but VA is the proper description). So, you want a 24V 72 VA (or greater) AC/AC step-down transformer. Here's a 100 VA one you can buy from ebay to get you started: -
You can feed it 110V or 230V so there are options here that hopefully will suit your country and you have two secondaries that can be wired in series to produce 24 V AC or in parallel to produce 12 V AC.
Finding the diodes and caps is easy
It's only easy when you know what you are looking for.