Speedo Europe has plenty of briefs with logos on the rear, mostly with 7cm sides. But they aren't licensed to sell to the US, so finding a seller to ship them here can be a bit of a problem.
For instance:
https://www.amazon.co.uk/Speedo-Mens...edo+logo+brief
But Amazon won't ship these to US. There are sellers who will, also look on eBay.co.uk.