From Citrus Recipes: Citron--the Oldtimer
Mercenaries of ancient Greece first discovered the citron growing in Persia. Its seed was spread from there to Palenstine, Greece, Italy and the Mediterranean Islands.
It is most notable for its chewy candied peel used in fruit cake and confections. Formerly cultivated in Florida and California, it is now grown commerically in California

From The complete Kitchen Guide
Citron: A citrus fruit from the Orient. A large fruit with a rough, uneven, thin yellowish-green rind. The edible part of the fruit is small and surrounded with a thick white inner rind. The juice is used in beverages, but citron is best known in its candied or preserved rind form, generally available in jars or packages and used extensively in fruit cakes.

From Joy
citron- a true citrus derivative which doesn't taste like one, at least in the processed form we find it packaged at market.

From Woman's Day Encyclopedia of Cookery
Citron--yellow-green and oveal-shape, it is 6 to 9 inches long with rough skin and a small amount of acid pul;. It is grown for its peel, which is candied.

Personal Note: some years back we were able to find it in Europe (in a small fruit shop in Belgium which specialized in preserved and candied fruits). It did not look the same as we find it here in the U.S. It was sold in a large chunk which was green and bumpy. Here in the U.S. it is hard to find except around the Christmas holiday. It is most often sold in a small dice in a small plastic container. It is often part of a premixed candied fruit mix intended for use in fruit cake. It lasts quite a long time in storage. Ironically, we often use it in breads we bake which we consider to be "European" and especially "German" ie Stollen or Santa Lucia Crown.

If I'm forced to substitute I would choose a candied orange peel (although the color is wrong becauce I find the lemon flavor too strong and domineering) The citron we use here is very subtle in flavor and is a pale green color generally.
Hope this helps.