With around 70,000 Crass Model Ithaca doubles produced, parts can certainly be found Lloyd. But just try to be certain about exactly what you are looking for... especially from sites like ebay, where many sellers tend to describe gun parts inaccurately. Just this morning, I was checking ebay for some Syracuse Lefever parts, and saw some items listed as Syracuse Lefever parts by a gun parts seller called "gun garage" that were absolutely positively not Lefever parts... not even close. Now, this is an ebay seller who specializes in gun parts, and who should know better, but messed up badly. There was a set of automatic ejector barrels with forend described as F Grade Lefever, but a quick cursory glance showed they had no dolls-head rib extension, they had a double underlug, and they had a six digit serial number. Any one of those things would immediately tell you that they were not from an F Grade Syracuse Lefever.
So at very least, check the return policy. And with Gun Parts Corp., formerly Numrich Arms Co., it pays to keep checking because they are always getting new stock in as fast as old stock gets sold.