"I bought this thermos for free" sounds perfectly normal to me. "That thermos was for free" Seems undesirable: I'd just say "That thermos was free." Agree with Jimi that probably the most suitable antonym for "free of charge" is "available for purchase." But, "obtained" or "priced" could do the job https://docs.google.com/spreadsheets/d/e/2PACX-1vThQYloKZWldp-SOKX4OjBXwRfrgWYaDRrLVem32lfKHmI8mCJiHgR2_0HftTtEvIaeLQv1auoUjdAn/pubhtml