Check with your local Jo-Ann fabric store. Thye offer discounts in the local Sunday paper and they are always running specials, plus they discount products for high volume sales.
Use some plastic plants & add in a few live flowers. If you get a plant in dirt, you can re-use it several weeks. Ask what is in season & that will be cheaper.
Online church floral displays are the cheapest sources. They provide you various categories and items under each category. The variety and options available there are vast than any other source.
It all depends what the floral displays are for. Your best bet it to go to a craft store and buy silk flowers and make them. During the year I suggest that you pick flowers up at garage sales. Vases would be a good idea to.
Flowers like Carnations are really affordable. Usually you can get them for under a dollar a piece and sometimes the florist will give you deals on multiples.