Something you notably missed, the Maleficium can also grant Magic Resistance. Still Infernal, but thought it'd be a good reminder.
As for other ways to get MR, I'm pretty sure Magic not having anything but Parma and Faerie not having anything at all was part of the design intent, as specified in the core rulebook.
Though I suppose if you like to consider indirect things, the Ars Fabulosa lets a person trade bodies with a Faerie temporarily, so if he needed Magic Resistance to accomplish a short-term goal, he could get temporary protection that way.
Just to be perfectly clear, Hermetic familiars. Canonically, generalized magic resistance for humans can only come from:
Hermetic magic OR
the Divine (in various forms) OR
the Infernal (in various forms)
I disagree. Note that it's not Summoning that provides magic resistance, it's an Infernal Art that does. A character with the Summoning Art, and no other ties to the Infernal or the Divine (and no Hermetic Magic) has no means to grant himself or others generalized magic resistance. And I think that even in the eyes of the Order the fact that Infernalists may have access to magic resistance is regarded the least of their ... sins
I referred to a full Summoner, shoulda been clearer; someone with full access to the Ars Goetia. And I disagree on the latter count, believe it or not; the Order as a whole doesn't really seem to care to go hunting down infernalists who aren't part of the Order. But an infernalist Summoner with Binding is on the Order's radar because of MR, since a Hedge Tradition with MR is going to have to Join or Die, and infernalists are likely to have difficulty joining (though apparently using infernal magic is not inherently Dealing With Demons, try to convince a Quaesitor of that...).
Any Infernal tradition is treated under the "Die" policy, not the "Join or Die" plan I don't think the Order will really care that an Infernal Tradition has MR; lots of them do, it doesn't really matter that much. Infernalists are generally much more scary because of the way they can use demons' deceptions, which Hermetic magic cannot discover, than because of high MR (or overt powers).
What will matter is if the tradition has exceptionally high MR, so that it becomes obvious that Hermetic magic finds it difficult to deal with them. That too will only matter only to the extent that it becomes an actual point, as in e.g. Hermetic wizards coming into direct conflict with them or their minions and failing to penetrate their MR.
But only well-established covens with access to the right Maleficia can manage high MR, IIRC. Through Ceremony, they can reach MR 150 or so. Using Binding to get a demon's Might or Ablating to get Infernal Might may grant you some MR, but demons have relatively low Might so that they'll gain, what, MR 30? That may pose a problem for some magi, but any magus with a focus on Parma can outdo that. It isn't even Parma-equivalent, let alone really-scary.