Search this forum for 'orphan foal", and you'll find some recent threads with a lot of suggestions. The biggest seems to be
1)find a nurse mare
2)if you can't find nurse mare, do the igloo and keep foal with an adult buddy that is kind. Best if it's a sweet mare who's had foals, but a kind pony gelding or even horse will work.
We successfully raised an orphan on the Igloo cooler. You can get Mare's Match milk replacer at MOST Agway stores. The one in Gettysburg usually has it, the one in West York does, too, the one in Carlisle needs to order it - don't know about any others. You need to make sure it is the powder for a newborn, there are pellets for later when the foal is ready for something a little solid.
But to answer the original nurse mare question, contact all the breeding farms in your area. Pennsylvania Horse Breeders Assoc. lists folks who stand TB stallions registered in Pennsylvania. There are also large Arabian, Standardbred & other farms around. They are most likely to be able to help, if you are not "hiring" a professional nurse mare (and they might be able to put you in contact with someone who leases nurse mares, if that IS the route you want to go.)
Hidden Echo Farm, Carlisle, PA -- home of JC palomino sire Canadian Kid (1990 - 2013) & AQHA sire Lark's Favorite, son of Rugged Lark.