Your solutions will depend on how much of a purist you are about such minor accessories.
If having the Real Thing matters, then you'll have to scour ebay and the like to find what you're looking for, and pay accordingly.
If however you, like me value function over form, you can simplify your life by taking the lens into a supermarket, drugstore or dollar store and check the shelves for a bottle cap, snap cap or the like that will fit.
Because I tend to use and store my lenses with the shades attached, I need odd sized caps, so over the years I've learned to be resourceful to find the funny-sized caps I need, and now I've got lenses capped with the snap cap from a can of tennis balls, the plastic cap used to re-cap opened soda pop cans, the plastic lid from a glass jar for holding dried spaghetti, the plastic cap from a jar of spices, and even the metal lid from a tin of baby's bum cream. Some may look funny, but they all work very well which is really all I care about
