Do a search on "boom vang". This is the loop that you are describing but it usually involves a couple of fiddles and a cam cleat.
Not a boom vang as far as I can tell from pictures. What I have is more like the "cleat for line to hold down boom" setup in the picture below (not my boat).
As far as a new sail, Ebay. There are also MANY sail lofts that specialize in used sails. I'd bet $100 would get you a nice one with lots of life left in it.
I hadn't thought about used. I'll check it out.
I tie the gooseneck of my boom to the base of the mast, through a pad eye, before I tighten the main. This allows me to tension the luff properly.
That's basically what's happening for me by virtue of the design. I hook the loop over the nub to stop the boom from riding up, then hoist until the luff is tight.
What might help you is buying a copy of Sailing for Dummies. No insult meant at all. I have a copy and studied for months before sailing. Great info written by sailing champions in regular guy language. A good resource.
I have a copy and read through it once a while back. I reckon now that I actually have a boat, I should give it another read.
As a trailer sailor, your needs are different that some others. This post has some great advice from knowledgable guys. Others I've seen are written by folks with big boats and very deep pockets. The things that will serve you (us) best is common sense and Yankee ingenuity couple with good sailing skills and COLREGS knowledge.
That's one thing I really like about sailnet - lots of nice folks who are willing to lend advice and experience, especially to a beginner like me.