They are rarely kept and bred, and even more rarely sold; your best luck is going to be contacting people who live where they do or going collecting yourself. Bugs of America will have access to several species from Arizona and is one of the best sellers to get them from.
Care is going to depend on your species, but in general they are kept a lot like mantids. Spacious enclosure, many climbing/molting surfaces, flies, moths, and grasshoppers as live food. Katydids also enjoy vegetables and especially fruit. What species would you want to get?