"I acquired this thermos for free" sounds beautifully usual to me. "That thermos was for free" Seems bad: I'd personally just say "That thermos was free." Agree with Jimi that essentially the most suitable antonym for "free of charge" is "for sale." But, "purchased" or "priced" could function as the https://docs.google.com/spreadsheets/d/e/2PACX-1vThQYloKZWldp-SOKX4OjBXwRfrgWYaDRrLVem32lfKHmI8mCJiHgR2_0HftTtEvIaeLQv1auoUjdAn/pubhtml