For a simple "around the layout" transformer, look on Ebay for a Lionel model 1033 or its slightly younger equivalent the 1044. They run about $15 to $30 in good condition. Maybe a member here will offer you one so that you can get going.
You'll want to arrange the wiring with a simple "either-or" switch or a couple of plug/sockets so that you will be able to choose which power supply to use. The modern trains are probably not compatible with the older transformer.