I'm not sure I've seen them other than ebay. You might try
http://www.helenhowes-sewingmachines...eadlebits.html and if there aren't any on that page - you might try contacting her and see if she might have some that aren't posted yet.
Another option is to have them made as
http://ismacs.net/sewing_machine_art...-machines.html Also two posts about someone who made their own
http://www.victoriansweatshop.com/po...5&forum=501752 and
http://www.victoriansweatshop.com/po...2&forum=501752
I'm guessing the irons have wheels on all four feet and two are missing. Maybe something that might be a little easier to make is a runner as shown at
http://www.treadleon.net/woodshop/tr...lerunners.html or a riser as shown at
http://www.treadleon.net/woodshop/tr...dlerisers.html and then shim the two missing feet so that it sits level.
Janey - Neat people never make the exciting discoveries I do.