Does anyone know a source of apple SEEDLING dwarf rootstock?

You can only get dwarf apple trees if they are grafted onto dwarfing rootstock. Seedling apples will grow to be full sized.

Some of the places that do custom grafting (look for "bench grafts") have hundreds of varieties to choose from.

Try looking at the list of varieties available at Wagon Wheel Orchards (2nd page of their blog). You can get hundreds of different varieties and your neighbors will have never heard of any of them. Eating apples, cooking apples, cider apples, all colors russeted or not, all sizes.

I just received Wickson Crab, Suntan, Knobbed Russet, Hawkeye, Belle de Boskoop, Swaar, Newton Pippin, Egremont Russet, Sun Crisp, and Kidd's Orange Red, among others. You won't find those in your local supermarket.

You could possibly find someone in your area that would graft your current trees onto dwarf rootstock for you, so that you could still have the same varieties.

The rootstock is named, but doesn't the fruit depend upon the bud that is grafted to the rootstock?

Maple Valley Orchards in WI: Our specialty is the hard to find antique varieties of apples including Cox's Orange Pippin, Early Joe, Fall Russet, Hudson's Golden Gem, St. Edmund's Russet, and Tompkins King. Currently, we have over 400 varieties of apples available.
Website: www.maplevalleyorchards.com

Interesting hobby, growing apple trees from seed and experiencing the results. I once read that the chances of getting a great apple that way is about 1 in 100,000.
You aren’t going to get any dwarf rootstocks from seeds, just isn’t going to happen. I think there is a connection with the tree you had that stayed small and the fact that it died. Sickly trees grow slow but aren’t dwarf.
All across the country, there are millions of apple trees either grown from seed or the graft was killed and the rootstock lives on. With your interest in un-named varieties, I’d like to offer a suggestion. Seek out the old homesteads, roadsides and backyards for some of these”wild” apples. Select 15 or 20 varieties that you like. Put a plastic tape/flag on those trees. In the winter, collect the new growth and have someone graft them onto dwarf rootstock. You’ll have unique un-named varieties, while improving your odds at getting good fruit.
The different varieties of dwarf rootstock come from layering. You can do it by bending a dwarf sapling to the ground and burying it. It will put out roots along the length. Just chop off a section that has roots and a dwarf rootstock will form. This method is done on lots of different plants.

Is it possible to dig up the root ball and transplant the trees? Not sure what you mean by "cut out the tree" but if they are cutting down the tree, can you move the root ball to a temporary trench? You can cut back the root ball if the roots are not supporting leaves and limbs. Keep them well watered and out of full sun.

You might get the tree(s) to throw another stem. If there is a graft, cutting above it may save the variety you want.

You're calmer about this than I would be. My area electric co. took 6 trees out of my side yard (not in right of way) and I went ballistic. To this day, the guys on that crew cross the street when they see me coming.

Luther Burbank would plant thousands of one kind of plant looking for the traits that he wanted. He would have people save their pits and would plant them to see what came up. Then select what he wanted to grow on and destroy the rest.

Plant breeding and development can be a fascinating hobby.

on the other hand, cloning thru tissue culture seems so sterile and mundane. But that's what makes the money.

Terri, You should check out Southmeadow Fruit Gardens and read their blurb on featured fruits. If that doesn't get you excited about some of the older named varieties I will be surprized. Cummins nursery also has a lot of great varieties. they also offer some trees on interstems ie MM111 rootstock/ G11 or Bud9 interstem. This gives a vigorous root system, drought resistant and good anchorage. The interstem produces the dwarfing effect and early bearing. I planted a Rubinette from them this spring.