What type of mulberry do silkworms eat, and where can I find this tree (I need leaves for feeding the silkworms)?
Silkworms will eat any species of Morus, though white mulberry (Morus alba) is their preferred food source.
I understand you are looking for a tree source. I wonder if you actually plan to plant the tree, or if what you really need is access to mulberry leaves. Numerous mail order nurseries carry Morus alba, Morus alba tatarica, Morus rubra, and Morus nigra, and in fact you may find local nurseries with trees, too–it’s just that they don’t keep an online inventory because it changes too frequently. You can search Plant Information Online for mail order sources. I did a quick search on Morus alba, and found several nurseries that carry it.
If you simply want leaves, you may want to check Arthur Lee Jacobson’s book, Trees of Seattle, which lists locations of trees in both private and public gardens. You would, of course, need to obtain permission to harvest any leaves.
This website offers information on real and artificial food sources for silkworms.