OK, it seems that you are very interested in that female. So, it is true that she is one of those most impressive female singers in America, and she has got a pair of blue eyes, so she doesn't need to get some blue contact lenses. However, due to some particular reasons, sometimes she would wear some contact lenses to improve her looks, or just make a change. So, the type of contact lenses that she wears might be the same as what we get.
Martina's eye color is originally brown, and she keeps her eyes as blue in her daily life. However, as a popular country singer and song writer, she has to take part in lots of party, and there are lots of photos and MTVs made for her.
Changing eye color is a quite normal behavior for her and for any celebrities. They need to make all the colors on their makeup matched.
If you want buy the same types like hers, you need to go to the glasses store, ask if they offer the cosmetic contact lenses.