3 Shovels can be bought with 250 beans. So as long as you have playing friends and keep sending them requests, it won't cost you a real penny.
You can only buy them BEFORE you hit play on a level though, otherwise it will cost minimum of 9 gold if bought as you are playing through a level.
You can only buy them when you have 0 shovels. I find if I am quitting a game then its better to buy 3 shovels first before quitting as they will regenerate and then I will end up stuck with just one. You get one free shovel every 6 hours as long as you don't have any saved already.
Buy them when the level screen opens
Click on the shovel to get this box
Other uses for shovels include :
Removing single crops
Stunning the Rabbit (my personal favourite. Its satisfying)
Moving a Chicken
Lighting a firecracker
Breaking a snowball
Lighting a firefly
Opening an oyster
Opening fireflies (or moving them on a stage)
Germinating a seed or moving it on a stage