I love mincemeat pies and tarts. My mom was English so we always had mince pie at Christmas time along with the traditional plum pudding with custard. It's not hard to find, just have to know where to look for it since it's not a common dessert for Americans, although I do think it has become more popular over the past few years. My local grocer carries it at holiday time but it's not stocked all year. I've also seen it in Walmart. My preference is Cross & Blackwell's with the rum and brandy already added. My mom always used to add an extra splash of brandy for good measure.
The englishteastore(dot)com carries a couple brands of jarred mincemeat filling, the Robertson's brand and another by Norfolk Manor. They also carry the already made mince tarts. Hope you find a brand you like. To me it's not Christmas without a mincemeat pie.