I've found them at the bigger fabric stores, like JoAnn's, but they're hidden in the back with the pressing supplies. Honestly, though? I buy most everything from Amazon.
If you can't find what you're looking for, the Notions Shop at my website (which connects to Amazon) sells a seam creaser and pressing ham, among other things. (Full disclosure: I get a very small commission off every item that sells.)
For more obscure (and expensive) items, check eBay. I recently bought a vintage clapper for a fraction of what a new one costs.
I Googled “Pressing Ham Holder” and came across http://fitforaqueen.wordpress.com/2011/03/05/pressing-hams/, which lead me to http://maryjos.com/notions/notions-ham-holder-772, which has wooded ham holders for $19.99 each.