I think AssortedBiscuits answered your question in the first couple sentences of their comment:
Most Westerners already hate communists and carry the grudge against the USSR. Anarchists don't really deviate too much from some generic Westerner.
It's really not any deeper than that. There's no need or reason to single out anarchists from any other average westerner when analyzing the source of animosity for the USSR because the answer is going to be the same whether you're talking about chuds, liberals, or anarchists. Even the non-western anarchists who hold a grudge against the USSR, the answer is probably still the same just because of the prevalence of western cultural hegemony all over the world. In your edit, you specify:
I'm curious about what information sources - mentors, friends, books, TV, cultural osmosis, conveys that information to people. Where do individuals encounter this information and how does it become important to them.
But the answer to that is the same information sources you yourself were probably exposed to early on. It's all the same shit we're steeped in, the ubiquity of anti-communism throughout western culture. Animal Farm and 1984 were required reading for me in junior high and high school respectively. The class discussions around these books were centered around teaching us that the USSR was corrupt, oppressive, and that these communist ideals that may sound like good ideas will always and invariably lead to "authoritarianism" and "totalitarian dictatorships" like the Soviet Union. Everyone absorbs that shit young, even the people who might later go on to question the truth of what they were taught, like anarchists.
You say
Newly minted Anarchists have to learn to hate Lenin and Stalin and whoever else they have a grudge against.
But no they don't. Not as newly-minted anarchists anyway. That brainworm software was already installed long ago before they became anarchists. A major part of becoming a leftist is going through a process of uninstalling all that brainworm malware. Anarchists who still hate the Soviet Union are people who have been successful at uninstalling much of the brainworm malware, it's just that they haven't completed the process by uninstalling the anti-Soviet or anti-"tankie" worms... yet. And I say all this as someone who long considered themself an anarchist.
You're right, there is definitely theory reasons too, but I think that's more general to states as a concept and doesn't do much to explain the specific grudge against the USSR or why there seems to be hatred for it that goes beyond states in general. There's historical reasons for that specific hate of course, which other comments covered better than I could, but I answered the way I did because of Frank's (OP's) edit about sources of information.
I think there's still another aspect for the specific anti-Soviet sentiment that has to do with many anarchists wanting to differentiate themselves from MLs or "tankies." Since we all agree we're on the left, there's a desire for a lot of anarchists to draw a clear distinction between themselves and those they perceive as adversaries or enemies, and strong disapproval with the USSR is a pretty obvious way to do that. I suspect part of that may in some cases come from a kind of "I'm one of the good ones" or "pick me" attitude, since they can say to liberals "yes, I am a radical leftist, but I'm not like those bad authoritarian tankies that we all know are the bad guys!" But the need to do even that I think has a lot to do with the general anti-communist milieu, that "malware" we're all indoctrinated with by default.