This is why it's so helpful to have a network of collectors to help each other out, and my preferred site is SportsCollectors.Net, or SCN for short. I don't know how many autographs I've been able to add or upgrade in my collection from that site over the last five or so years, but I can comfortably say that number is in the 100s. Whether via private signings, trades, consignments, or 50-50s, it's amazing what a country-wide group of collectors can do to help each other out. I highly recommend the site for any other autograph collectors out there. I think it's only like $15/year, and while you still run into some scumbags/shady dealer types, by and large the feedback scores will largely give you an idea of the type of collector you are working with.
Here are some of the autographs that I've been able to add or upgrade using SCN over the past six months or so. Not too bad.