Did a web search for floor jack adapter and found http://www.mrtool.com/browse.cfm/4,1359.html Looks the same as the one from Eastwood but this one is only 7.99 I don't know what the quality of it is.
See they are in DuBois, PA so I'm sure someone here on the forum may know where it is. It's about 120 miles from me and my sister-in-law lives close to that area.

See there is also a store in Hubbard, Oh, and one in Angola In.

 

Damn!!!! That's the exact same one I bought, for a lot less money.

I just did another search with quotes around floor jack adapter and the one you found showed up on page 4 of my Google results. Without using quotes, it showed up as of page 23..that's why I didn't see that one.

 

Seems easy enough to make that adapter out of a piece of lumber.... and reinforce it with 2-3 bolts across the length. Time to go dig up something out of my kiddling wood box. I'll post pictures if I end up with something working well. I've been thinking about making that adaptor for a while; I'd use it to lift the entire side of the car during a seasonal tire changeover.

Yves