The only sustitute is a hazelnut liqueur.
http://www.webtender.com/db/ingred/167
Frangelico taste is achieved by infusing toasted hazelnuts in alcohol and
water, then distilling the result. Other ingredients are added - including
toasted cocoa, toasted coffee, vanilla berries, rhubarb root and sweet orange
flowers - before further blending and maturing in oak casks.