https://www.library.illinois.edu/ias/spx/slavicresearchguides/subjectresources/subsourbul/lingbulg2/
https://www.library.illinois.edu/ias/spx/slavicresearchguides/subjectresources/subsourbul/stoikov2/
https://www.library.illinois.edu/ias/spx/slavicresearchguides/subjectresources/subsourbul/balan2/
https://www.library.illinois.edu/ias/spx/slavicresearchguides/subjectresources/subsourbul/lingbulg2/