If Google Books has it in EPUB, you should be able to get it from B&N. Some Google Books are only available in raw-scan PDF, in which case you'd have to get them from Google. But then you have to deal with the NOOK's poor PDF handling, which probably can be characterized as abysmal for scan-type PDFs.
For EPUBs, Google Books tend to be rather poor quality, with lots of OCR errors. You might want to check out Feedbooks, Manybooks, BeBook, or Project Gutenberg first.
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