Since the sockets are rusty, you'll need to replace the whole units. There are now solid state clearance lights that use LED's, with no provision for incandescent light bulbs. The better ones are potted and are thus very thoroughly waterproofed. Check around online for those, such as on eBay, Amazon, and truck and trailer accessory web sites.
Or, you can just replace the few that are rusted with new ones that take 1157 bulbs, and then put in LED bulbs. That's a viable and time tested way too.