Does anyone know of a good collection of Band in a Box bluegrass songs?

Aidonithith, firstly, welcome.

You mention Songs and not Styles.

There are quite a few Bluegrass songs in the C:\bb\Demos\RealTracks - Demos\ folder. These are demo songs produced by PG Music.

The PG Music Band in a Box Radio channel also has a search list for Country, which will included Bluegrass.