There are a number of solutions like the one you're seeking, though from my experience, they don't stand up as cost effectively as the Elgato systems, as they're aimed at pro video producers and the like. Also, I'm not sure if their software allows you to view the feed from the input as easily as the Elgato system.
By all means, however, check out the Apple Store's page of video I/O hardware. (
Link)
Apologies if the UK store is irrelevant to you, but it's my default, and I feel sure that there are equivalent products on the worldwide market.