You can find plenty on ebay. Sorry, but you're a fool to go to any non-ebay dealer for brand new release films. You'll pay double for nothing.
Now, older films are a completely different colored horse. But lets' remember - just because you see the poster in person and the guy tells you it's original doesn't mean it's original. Most likely, he bought it from somebody, and they told him the same story. It all comes down to trust, and building it over time with a particular dealer.
I have four or five that I use regularly on ebay, and they are all trustworthy for new movies. They get their posters from the same source as the theater, which is common these days. Some companies frown on their posters being sold (Sony is one, I believe) but most couldn't care less.
I'll use some recent examples. I picked up that Rocketeer teaser I mentioned a couple pages back. I came this week and I'm very happy with it, it's double sided, heavy stock, exactly 27x40, with a VERY high gloss finish on the front. Is it original? Probably, as all three of these things point in that direction.
I also bought a poster that means something only to me and my wife - Short Time. Remember that one? No, I figured you didn't. It's a flick from around 1990 with Dabney Coleman, and it was the first film my wife and I ever saw at the theater as a couple. I'm currently putting up about 16 posters in a long hallway in my basement, and I wanted to frame this one as something special to us. Anyway, I bought a single sided folded version for about six bucks on ebay - hey, this isn't exactly going to be a big seller. You can often buy second rate flicks like this for very little cash, since they don't have a lot of broad appeal. The guy claimed it was 27x40, which is potentially correct for 1990.
Alas, it's a reprint. It's 27x39, a sure sign that it's not original. But I'm out less than ten bucks, so it's no biggie. I wouldn't have gone with someone I didn't know for a $100 poster, but for ten bucks it was worth a shot. I'll start the hunt again.
Another really nice poster that I picked up recently on the bay from a dealer I've bought from before is Beauty and the Beast. This is the one with the very colorful cast in front, and the beast looking down from the clouds in the back. Great artwork. The guy said it was ds and 27x40, but when it arrived, I was pleasantly surprised to find that yes, it was ds, but it was also 27x41! It's the only poster I have like that, one that was caught in those transition years between ss to ds and 40 to 41. I thought that was pretty cool in itself.
If you do buy off ebay and want some names of people I've had luck with, I'll be happy to provide. Also, there's some key things to watch out for - if the seller says anything about the size not being exactly 27x40 or 27x41, like 'size could vary by up to 1"' for example, that's them covering their butts on a reprint. While I believe there are some originals that aren't exact (or within no more than 1/16th off), they are pretty rare.