I usually buy mine at Di Palo, where the grey mullet roe (rather than the tuna), vacuum packed whole lobe, is about $90 a pound these days, or about $23-$30 a "lobe", depending on the weight. I've also seen it at Buon Italia in Chelsea Market, at Dean & Deluca Prince St. (VERY expensive), and a Spanish bottarga at Despana on Broome Street - I don't remember the price off hand, but I think it was in line w/ DiPalo.
I find that the one lobe lasts quite a while for us - maybe three pasta dishes for two.
Di Palo's Fine Foods
200 Grand St, New York, NY 10013
408 Broome St, New York, NY 10013
75 9th Ave, New York, NY 10011
Dean & DeLuca
560 Broadway, New York, NY 10012