When you say “you would love to find a source to propagate off of” . . . are you talking seed or cuttings?
I have 11 Juniperus rigida in Anderson Flats. They are several years old however, I haven’t noticed any juniper berries as of yet. I may have to put a few into the ground and/or wait a few more years before that happens. My understanding is that Tosho are poor movers of water. I’ve assumed that might make them difficult to propagate via cuttings or layering (but I haven’t tried). Sheffield’s still has Juniperus rigida seed available:
Juniperus rigida, commonly known as Temple Juniper, is a beautiful and attractive evergreen tree, native to northern China, Korea, Japan, and the far southeast of Russia. It has a loose, open habit, with sharp, needle-like bright green foliage that looks
sheffields.com
I second the vote(s) on Pinus parviflora, Crataegus laevigata 'Paul's Scarlet', and Fagus crenata.
The U.S. bonsai community would benefit from a good source of Prunus spinosa. A few years back, I managed to source a few that are now producing lots of fruit (seed) and they now have plenty of branches & roots for cuttings and layering. I had a hell of a time finding the trees I have. Trees and seed are banned from import into the U.S. due to Plum Pox Virus. I stratified some of the fruit that I harvested last season but, I didn’t take the time to scarify the pits. I have grown them from seed in the past but, I cracked the pits prior to putting them into refrigeration and had success the next spring.
Perhaps I can get you some propagation material if you have trouble sourcing viable seed or live trees.