I got mine on Amazon.
Shepherd Textiles Tussah Silk...
https://www.amazon.com/dp/B08RRFLY93?ref=ppx_pop_mob_ap_share
It has lasted forever!! I don’t really measure it. I just cut about a dime size amount and add it to the lye. My soap batches are about 55 oz. If you cut too much I have found it doesn’t want to dissolve as well. I add it right before I stir the lye and water, because the heat dissolves it well. I definitely don’t want strings in the soap. Customers won’t know what that is, so I cut it up really small. That one container has lasted me a really long time.