Least expensive approach is Lionel Powerhouses (180 watts each), which have good built in circuit breakers. But this doesn't allow for conventional running, and are suitable for command only layouts (e.g., PS2/3, Legacy/TMCC, LionChief/Bluetooth). If you need conventional capability, something like the Lionel GW (180 watts) or ZW-L (620 watts) will provide plenty of power for most layouts. These are relatively expensive, but most people who have them are pleased.
Another approach if you feel comfortable with things electric is to buy older transformers that have been reconditioned or recondition them yourself such as the old (as in 70 or so years) ZW, and add modern circuit breakers to the wiring. I don't recommend that unless you are really, really comfortable with electricity and circuitry. I would also add transient voltage suppressors to the wiring for any older transformers.
My own choice would be Powerhouses for command only layouts and the ZW-L for large layouts with conventional.
You an also buy used MTH transformers (e.g., Z4000) but these will be without warranty and are older designs for which repair parts may or may not be available in the future. The MTH spin-off company has promised continued development of the Z4000, but right now they don't have a customer service function, email address or website, so there is uncertainty.
