Yeah, I think you're right. So probably the best bet is call the manufacturer first, and if they won't ship it, get the part number, then go to your LFS and ask them to special order it for you.
Well there you go. 18 bucks is cheap insurance against a broken 55g tank and the expense of 55 gallons worth of water damage and subsequent fish deaths. Although it looks like that's a replacement top frame for the tank. The center brace is one piece with the rest of the frame.