Ultra dwarf citrus is likely to be grafted onto FD rootstock.
There's also the California Citrus Clonal Protection Program, which you can order graftwood from. Then you would have to try to root it. (Hopefully doing your research first so you don't waste your time and money)
You can also order seed for rootstock here, but they are mostly wholesale and do not sell in smaller quantities:
http://www.citrustreesource.com/Products/Seed.aspxSince you are in California, it's more difficult and expensive for small businesses to operate because of all the regulations, so that's one of the reasons you are not going to find lower prices (in case you were wondering and wanted my opinion).
I actually found Flying Dragon for sale at the local nursery, but in WA state.