You might want to try these folks for a full line of wire, bulbs, and more:
Miniature Lamp Works
The owner has been providing toy train bulbs and wire for quite a few years. I think her prices are very good. I haven't placed an order in a while (she is at York, in the Orange hall), the last time I did, the turn around time was very good.
Lionel used both plastic covered flexible wire and cloth covered solid wire for the runs between the e-unit contacts and the motor. Different color insulation was used too.
Same goes for the wire from the pickup terminal to the e-unit terminal, and so on.