Try H&L's bait in Morrisville - if my memory serves they carry shiners of all sizes including some as big as 8" (I know they carry 6 inch shiners). They are located on bridge street in Morrisville right next to the Delaware River - less than 1/4 mile from where Route 1 crosses into NJ - lees than 45 minutes from Philadelphia. Their number is 215-295-1400.
Hope this helps!
Butcher