Research which plants you are interested in growing and find out what diseases most commonly affect them. If you have experience with an organic garden, you may already have a pretty good idea.
Look for nurseries that sell organic plants. You can buy disease-resistant plants at most nurseries, but not all of them offer organically grown plants. If you are going to have a true organic garden, you need to start with plants that have been started with organic seed and raised organically.
Identify which varieties of the plants you want have disease resistance. Some varieties or cultivars will be more resistant to disease than others, but there may be a trade-off in terms of plant size, produce quality, heat or cold tolerance, size or cost.
Make a note of any plant diseases you have had trouble with in your garden in the past, or that are common in your area. You should look for plants with a heightened resistance to these diseases, to increase your chances of success with that type of plant.
Purchase the plants you feel will work in your organic garden. If you can't find what you need locally, there are many online and mail order nurseries that specialize in various kinds of plants, including those that are both organic and disease-resistant. The plants you get should have far fewer problems with disease than those types that are not disease-resistant.