They usually sell in major sports memorabilia auctions. T206 Honus Wagner cards are often the centerpiece of those catalog sales, but if you're an eBay member...and have a few hundred grand sitting around...you can avoid the wait and buy one on eBay.
The card has been bought and sold via dealers for a number of years. It isn't known who the most recent owner has been.
While the price may seem steep, it's still less than one collector paid for a T206 Honus in the spring of 2009. Robert Edward Auctions sold a higher quality '1' for $399,500 in its annual event.
Other recent Wagner sales have included: $219,225 for an authentic, but altered version, $925,000 for an SGC 40 (VG), and $1.62 million for a PSA 5.
Here's a link to the listing which is set to end late this month.