That thread is a great start.
My favorite place that sells rootstocks is:
Fruitwood Nursery - Rootstocks
They often mention which are started by seed or otherwise. Buying rootstock trees is one way to get started with propagation of those that are normally started from suckers or cuttings.
If you want to look deeply into the characteristics of individual trees used as rootstocks, here is one website.
UC Davis - Foundation Plant Services - All about rootstocks
This is the sort of info you can find, as an example from the page I linked:
Cultivar Name: Myrobalan 29C
Type Plum Rootstock
Patent Not Patented
Parentage Open-pollinated seedling of Prunus cerasifera
Species Prunus cerasifera
Usage Rootstock
Scion Vigor Influence Standard
Soil Adaptation Good tolerance to waterlogging. Has adaptability to medium to heavy soils.
Resistance Traits Resistant or tolerant to Root-knot nematode. Susceptible to Lesion nematode, Ring nematode, bacterial canker. Partly resistant to Phytopthora, Armillaria, crown gall.
Graft Compatibility Good with Apricot, Plum, "Prune", poor with peach and nectarine; some incompatibility with almonds; prunes subject to brown line on this rootstock.
Propagation Notes Its propagated vegetatively by hardwood cuttings.
Propagation Method Vegetative
Anchorage Good
Suckering Medium
Description Originated in Marysville, California, by Marion E. Gregory, Gregory Brothers Nursery Co., now of Brentwood, California. Introduced in 1920. Open-pollinated seedling of Prunus cerasifera; seed imported from France. Used only as a clonal rootstock; resistant to root-knot nematode, grow vigorously; makes large tree; less sucker development than Marianna 2624. Not in NC-140 Trials. A standard rootstock for plum in California, especially in heavier soils where Nemaguard does not do well. Disadvantages of Myrobalan 29C is it tends to lean, some incompatibility with almonds, prunes subject to brown line on this rootstock, may set lighter crop than Marianna 2624, susceptible to oak root fungus.
This is a little simpler, it's a list of common rootstocks and their characteristics:
Dave Wilson nursery rootstock comparisons
Have fun!