Sapa, in Latin, or Siraion (σίραιον), or Hepsema (έψημα), in Greek, is a grape syrup made from grape juice by boiling it and reducing it to 1/3 of its original volume.
In the US, most grape juice available on the market is made from American grape varieties, such as Red Concord or White Catawba, so it is not authentic to ancient Roman or Greek recipes. However, a good brand of Concord grape juice which reduces well is Kedem. It is available in most supermarkets kosher sections. Another option is to simply buy grapes and squeeze them into juice at home. Authentic Roman grape varieties today are only grown in Greece and are primarily used for wine production, so they are pretty much off limits to most people.