+2 on Ruger, at least. I ordered GSR mags back around the first of the year when the new 10 round poly mags were out of stock, but on backorder. I got them 4 weeks later, which is sooner that I expected.
This is good advice, if the vendors will take backorders. Another is to just monitor the online vendors regularly, and pick up what little you can when you can. It will add up over time. I've been slowly building up a cache of .308 ammo since I bought my Ruger .308 on Dec 30. Forty rounds here, a hundred rounds there. I've probably acquired 600 rounds or so that way since the first of the year. I have a personal goal of keeping 1000 rounds on hand for each rifle I own (5,000 for an AR or AK). Back after the first panic in 2009, when ammo was still on the shelves at the big box stores (in my case WM and Academy) I'd go in once a week and pick up three boxes of either .223 or 7.62x39. Over time, I ended up with plenty of ammo in these calibers. Can't do that right now, as it either isn't on the shelf at all (WM) or there is a one box limit (Academy). But that will pass, and when it does, I'll start picking up some more .308 that way. Meanwhile being patient but attentive pays off: today I should receive 100 rounds of Herter's .308 from Cabela's that I ordered a week ago at $12 per box of 20. Of course, it is all gone now. But who knows what I'll find this week, or next? The tortoise wins this race.