From from 'History of That Part of the Susquehanna and Juniata Valleys Embraced in the Counties of Mifflin, Juniata, Perry, Union and Snyder', Franklin Ellis, Philadelphia, 1886. Alexander information begins in Chapter XII on page 617. Here's just a snipet of some of the invaluable Alexander family info found in this book::
"One of the first to locate in the valley of the Kishacoquillas Creek, was James Alexander (1726-1791). He was a son of James (actually John 1700-1777) and Margaret (Glasson) Alexander (1700-1777) native of (County) Armagh, Ireland, where he was born in 1726, and when ten years of age, emigrated with his parents to America and settled in West Nottingham, Chester County. From there, the family moved to Cumberland County.."
The chapter goes on to reference their children and descendants and where they all migrated to from there. This publication is online. A Google search for 'History of Mifflin County, Pennsylvania' should take you to it.