I would also suggest ebay, they usually show the actual pictures of the plants and are way cheaper. Just look for a seller with great feedback and their plants are usually pretty good quality. At Petco you pay 7-8 dollars for a single large (16 to 20 inch plant) while on ebay they average about 3-4 dollars per large plant.
If you don't have the time or the resources for real plants. just go with plastic, thats what i did. and though they don't look as good as real plants, you can still use them in a way that would make your tank look good. here is a pic of my tank. hope it gives you ideas.