When I bought my land, I had spent almost a year looking at different places with a realtor. He was finding them in the MLS, and if the land was interesting, we would check it out.
After awhile, I started looking online for listings that where not in the MLS system. For some reason that I cannot explain, I bought a Sunday Newspaper one day and I found my place listed in the "Land for Sale" classifieds. No MLS, just the size of the land, the address and the price. I drove out there on my own, liked what I saw, but it didn't have a good entrance. I talked to my realtor, he found out who owned the land next to it, and called them. It was a company that wanted to get rid of the land, but hadn't listed it yet. We agreed to a price, and I bought both properties, then a year later, sold off the part with the bad entrance, which paid for what I kept.
If you cannot find what you want in the MLS, look at the local papers, and sometimes, just talking to people in the area might lead to a place that's for sale, but not listed with a Realtor.