How to keep cream from separating in milk?

A very quick option is to buy less milk at a time - unless there's a very specific reason to by in bulk, picking up a quart only when you're ready to use it may solve the problem, since it seems mixed when you buy it. Additionally, raw milk has a shorter shelf life than pasteurized, so buying fresh may be better anyway.

If you did have a reason to buy in bulk, you might try mixing the whole (separated) quart when you first open it, and perhaps every few days when using. Shaking may take more effort, but pouring into a container and using, say, an immersion blender may very quickly mix the milk well enough to keep for a few days while using.

If you get hold of a little milk frother (works kinda like a very very tiny immersion blender) it may be useful to mix the milk still in its container, at least as long as the liquid is high enough to be reached.

Another possibility, one I've no idea if it will work or not, is that depending on the shape of the milk containers, you might be able to stand them on their heads, say, every other day. Cream rises upwards, having that upwards change direction every so often might keep the cream in suspension longer without requiring a lot of physical effort. I've heard it works to keep peanut butter from separating (even on longer timescales, flipping once per week or month), but then peanut butter is so thick and the timescale it takes to separate so much longer I'm not sure if milk will work the same way.

I don't recall how long this kind of mixing will stay un-separated - I tended to shake just before use and that was effective enough for me - but it may help, even if you have to do it periodically.