Yo may wish to search the catalogue of a Bulgarian library such as Bulgarian National Library.
You will need to be able to type in Cyrillic. It is quite easy to instal a Cyrillic keyboard onto a computer these days. However, if you do not have access to a Cyrillic keyboard, click here for a Bulgarian keyboard.
Bulgarian search-engines include Google (Bulgaria).
Works of Bulgarian literature and books about Bulgarian language and literature are to be found in the Research Collection of the Taylor Institution Library. This library also has a good selection of Bulgarian Language and Literature periodicals and reference material.
In order to locate books on the shelves you will need to find out the shelfmark by looking your title up on SOLO. You will need to transliterate according to the Library of Congress (LC) Transliteration Scheme if you are looking for a Bulgarian title. When you have looked up your title you can take it from the shelves in the Research Collection in the basement.
There are two classification schemes at the Taylor Institution Library. New books in the Research collection and the undergraduate collection are classified according to a standard version of the LC classification scheme used across the Bodleian libraries. Older books in both collections are classified according to inhouse adaptations of LC. LC Slavonic shelfmarks begine with the letters PG. They are then numerically subdivided by language and literature. The Bulgarian Church Slavonic sequence begins PG771-799. The Bulgarian Language (and Literature) sequence is PG801-1146.
If you come across a shelfmark beginning TNR or REP. SLAV (or no shelf-mark at all), this means that the book is kept out at the Bodleian Storage Facility (BSF) and you will have to do a stack request.
If you cannot find the book on SOLO, you could try looking for it on JISC Library Discover which is a union online catalogue of UK national, academic and specialist libraries. WorldCat covers libraries throughout the world and includes millions of records representing 400 languages. KVK (Karlsruher virtueller Katalog) is another union catalogue covering academic and national libraries throughout Europe and beyond.
The UCL website has a good selection of Bulgarian e-resources.