Pinetree Garden Seeds has sunflower seeds, 20 seed per pkt. for less than $2.00 with a lot of varieties. www.superseeds.com
We have bought from Pinetree for years and have never had a problem with them.
Zinnias are a good flower to attract butterflies and they do well in dry or neglected situations and are easy. Marigolds are also bright and easy to grow. Cosmos are a good flower to broadcast and grow a 'field' of flowers and are favored by bees. Sow in the fall or during a wet spell and you won't have to dig either. They often will self-sow as will some marigolds.
Those are some of the standard, easy and lower priced flowers you can sow.