Where can I find these two items?
The Bazaar is the best place to obtain a mandrake at your level. You can find them under the “Pets” tab. You can buy silk there too, but it’s better to find it doing odd jobs around Spite.
Both can be purchased in the Bazaar. For the Mandrake, that’s probably the best option, as the only other way I could find is at Bullbone Island, which doesn’t open until very late int he game. Silk Scraps can be found in many places, depending on your Qualities. I’d suggest poking around one of the excellent wikis (http://echobazaar.wikidot.com/ is my favorite) to look at the rewards for actions available at your level, or if you let us know approximately what your Persuasive/Shadowy/Watchful/Dangerous are now, someone might have a good idea.