If you're confused about what biga is, don't worry too much. You can just use the yeast they have at the supermarket in a pinch.
It probably won't taste as good as bread made with a traditional biga that's been prepared in an old fashioned way, but it will still rise the way you need it to.
Even if you do get a traditional biga you might want to consider occasionally adding a bit of supermarket yeast just to make sure the yeast organisms remain strong. I find they can go down in quality if there is no added yeast from outside sources for a while.