I too have had difficulty in finding my favorite root stock seed but I got some this past summer and am using them now so I looked them up. Their website will not lead you to the seed by requesting tomato root stock in the search bar. I finally got it to come up by listing the seed name. Below is a link to the site. You can buy from 50 seeds on up. I now buy the 500 lot so I have plenty for two years at a time in case of a problem like I had this year where all my early seeds died in the greenhouse due to the darkness caused by too much rain and cloudiness all through January and February here. Luckily I had plenty and could start over but it has delayed my planting by quite a lot since I try not to plant any non grafted tomatoes in my garden due to the awful fusarium and RKN problems that I have. This variety also gives some protection from Bacteirial Wilt which can be devastating some years.
https://www.neseed.com/shop/vegetabl...ato-rootstock/
I hope you are able to get the seed at least in time for a some late summer tomatoes and make sure to get enough for next year just in case.
Bill