Doug's is carried by most parts retailers like Summit and Jegs. AutoZone also sells them, every now and then they have 15% or 20% off sales for online purchases which you can use. I bought a set of ceramic-coated D453's for my '70 Duster back in November when they were having a sale.
Some have had issues with AutoZone being out of stock or shipping super late, I had no issues myself but one way around that is to have Summit price-match the sale price from AutoZone and just buy it through them instead.