If you can't get alfalfa hay, you can feed her a bit dried herbage every day - that is rich in calcium too.
The extra vitamin C and calcium are for the pups - the calcium is obviously meant for the bones. However, a mum on a good normal diet shouldn't have any problems giving birth do well developed babies, so don't panic!
I started my adult piggy life with a pregnant shop sow and had two healthy pups without the backing of a forum!