If you have a local hobby shop that does trains, most usually have quite a few transformers either used, or that came with starter sets...usually for around $5-$15.
I bought a $13 90 watt Lionel 1033 for 16 volt duty, and an old AmericanFlyer 100 watt transformer for about the same. This powers accessories that like 11-12 volts. If you're concerned about using only D.C. for "sensitive" LEDs without bridge rectifiers, an HO gauge transformer may be just the ticket.
Tom