Hi,
FYI - Chernushka is nigella seed.
You need to look for bakeries featuring Russian breads, though no guarantee it will be there. Garden Fresh has quite a few Russian breads, especially the one on Townline Road very close to Alef. If you don't see what you want, at least take note of the bakeries producing these Russian breads. What you want they may still make, though the store you are visiting doesn't necessarily order it. You may want to call them to see where it can be purchased.
I would call:
Alef Sausage & Deli
356 Townline Rd
Mundelein, IL 60060
(847) 566-0008
alefsausage.com
Levinson's Bakery
2856 W Devon Ave
(between Francisco Ave & Mozart St)
Chicago, IL 60659
(773) 761-3174
These people are from Georgia, though they may know a resource:
Argo Bakery
2812 W Devon Ave
Chicago, IL 60659
(773) 764-6322
Please report back on what is found.
Regards,