The common names of 'filbert' and 'hazelnut' are interchangeable. They simply refer to the fruit of any Corylus species. Filbert tends to be a European convention; hazelnut is the American version :-)

Without knowing which species you have growing now, I'd opt for a self-fertile selection that will at least allow for some sort of nut production. These are relatively common in the trade and include many hybrids. But many species/varieties of corylus will cross pollinate at will so I doubt you need to get too picky! The ornamental Corylus avellana 'Contorta' or Harry Lauder's Walking Stick will produce nuts readily from time to time that I assume are cross pollinated by our native Corylus cornutas.

From the description, it sounds like your bush is a seedling from an unknown cross. Because of that, you probably will not need to be too picky about the cross pollinator. Also because of that, you do not know what the quality of the nut is going to be. I would suggest that you find a source for named variety hazelnut bushes and choose one of those. That way you will have at least one bush producing quality nuts.

Thanks for the advice!
I am looking at the Raintree catalog - they list several Filbert Blight resistant cultivars - like Jefferson, York and Yamhill, which are listed as hardy to zone 5 (and I am in zone 6) - does anyone have experience with these varieties in the East? Would my bush have a reasonable likelihood of being able to pollinate those?

Here is a link that might be useful: Raintree catalog - Filberts

Look for the cultivar 'Dorris' - blight resistant and a suitable pollinator for just about any other corylus variety. Available through Burnt Ridge nursery.