Here's a tip for you if you are looking for ones that aren't fiberglass. '61 Seneca skirts will fit all 60-61 Dodge Dart and Plymouth full size models (all except Valiant). And in 1960 stainless skirts were available for the Plymouth which would fit your '61 Dodge if you wanted to go that route. If you search for 60-61 Plymouth skirts on eBay you'll have more luck finding a set. The stainless ones go for a TON sometimes (I paid $250 for a set, I've seen them go as high as $700) But you can often find original painted steel ones for around $75-$150. Fenderskirtking as listed above has them in stock but they're usually fiberglass, so keep that in mind.
The Jalopy Journal
