Try LBS Horticulture for baskets. How about a recycling system for the baskets, where if your customers return the basket once it's finished, they will get the price of it off the next one they buy
I like the idea of something unusual such as the trailing tomatoes mentioned, but also herbs look good and a hanging basket can put them right near the house where they can easily be picked. Something like that would be an extra draw for people to buy from you, rather than from the next place. At the other end of the year you could have winter baskets with violas, pansies, trailing ivy, small heathers, etc, with early crocus ready for spring.
If you are good at propagation and live in a warm area, the cheapest option for plants is to grow your own, and you get more variety, but it is quite fiddly and weather reliant.
For what to plant, as well as those things mentioned, there are petunias (I don't like them but most people do), bidens and various single colours such as the new trailing, small flowered red begonia. Have a look through the plant plug catalogues to see what's available.