Somehow I fell into the routine, motivated by wanting to see communities grow on lemmy, and I've just kept going.
You'd be surprised how quick I am at it now. The actual posting probably only takes 5-10min per day, and then whenever I have time to check my phone, I'll catch up on my inbox.
I mostly post from my phone, using the Voyager app. You can star a handful of communities as favorites and keep them at the top of your list, so it doesn't take a lot of scrolling to find the right one.
Do that a few thousand times over the past year or so, and you get pretty fast at it!
God, all over. You name it, and I've probably found content there. Random stuff from my Google recommendations, topic searches, reddit (rarely), Twitter/X, Tumblr, imgur, memes my wife sends me, YouTube (some people actually post memes there), etc.
Pretty much anytime I see something that makes me laugh or I think Lemmy would enjoy, I add it to my (now massive) vault of memes and images.
And if I'm feeling inspired (or bored), I'll sometimes make my own.