Rich this has caught me out many at time and know the place your quoting saying medium etc. Extra toolposts you will be limited to buying from them. Having bought extras in the past none fit my 250 series and they don't know themselves, though measure up within 1/16".
Far better to go to another supplier that products ties in with everyone else and can be readily bought. A 250 series will be a little small I would opt for the dearer 300 series. What that means is if tool post says 250 or 300-xxx you look around for those holders from any supplier often at substantially reduced price. I get most of mine (27) from Gloster in 5's or ebay and need more.