You can get 5 alternating flasher module kits for $8.85 shipped from China (eBay item number:165117958891)
But you would need to adjust values of resistors and/or capacitors to change the rate.
For those who don't want to take the time to assemble a board, for $8.99 plus $2 shipping from California you can get 2 adjustable rate alternating flasher units (eBay item number:392900420800)
Bob