They're pretty easy to make yourself, in a pinch. Especially if you have access to a lathe.
Otherwise, any welding supply distributor can get them.
Tim
call the people you bought it from and ask them where the rest are unless you bought used but yeah any lws should be able to help you
If you did buy it used just find a guy who has the spacers get some 1/2" cold rolled SS and some SS bolts and make your own.
i bought a h&M machine used it onl came with the 4'' dogs i had to get a set thru my lws they told me they couldnt get indivdual dogs just a full set and it was 65.00 that was 2 years ago i have seen them made with regualr bolts but was toild it was a pain in the neck to get them done
I have seen these new dogs that are all in one. they look like little steps. 40 bucks I have ordered them but have not recieved them yet. i used some bolts once but it took to long to adjust each bolt to get the machine square with the pipe and it still wasn't a real square cut
Those are called "Cuboid Spacers".