Does anyone know of any online websites that sell host plant trees/shrubs? moderator's note: added a more descriptive title to topic
What you need to do is determine which butterfly you wish to attract to your yard...research what plants are the caterpillar host plants and nectar source plants for that butterfly...then do a search by plant name. All online nurseries will sell a wide assortment of plants for attracting butterfies, just be sure the butterfly and it's plants are common or native to your area.