Answer
Go to a grocery store florist the day before or the morning of your wedding. You can purchase very inexpensive flowers there, just buy the per stem flowers, not an arrangement, because you can arrange and cut them yourself (you'll probably have to cut the stems really short to arrange them on the cake). Grocery stores have really great prices for just the raw flowers (like $0.25 each) and I can't imagine you would need more than 10-15 flowers for two small cake arrangements. If you cut and place them on the cake the morning or afternoon of your cake they should last the day (until you cut the cake). It will only be 5-7 hours, so they should last if they are fresh. Good Luck!