If you’ve been waiting all year to cross items off your wish list, or want to get your holiday shopping over with, today’s the day. Black Friday sales are officially underway and retailers are offering some of their lowest prices of the year on tech, apparel, home goods and more. But the absolute best Black Friday deals won’t last long — as a reporter who’s covered this sale for five years, I can say with confidence that many brands sell out of their top sellers quickly. My number…