Hi @M Kuzma
Here’s the [https://www.manualslib.com/manual/1557139/Sony-Ht-Ct260h.html?page=1#manual|service manual]
Haven’t had time to review it but but has the schematics so you should be able to work out how the power gets to the regulators on the mainboard to provide the different voltages and also what type of regulators they are. Once you know the regulator model number, search online for replacements using the model number of the component. Search on places such as mouser.com, digikey.com