• Redhouse: The best source by far, in English, contains pronunciation information, multiple spellings leading to the standard one, and a single word will contain multiple definitions, many being contemporary for the time. Though it is best not to use it directly from the archive.org book and use an online version such as in https://ingilizce.cagdassozluk.com/kamus/ or in the below aggregator.

• https://www.osmanlicasozlukler.com/ : This combines all well known Ottoman Turkish dictionaries in one site with search functionality (including Redhouse), searching is a little problematic. Not all terms have been transliterated to Modern Turkish, not all terms use the same letters, the main culprits are:
  • ك/ک/گ/ڭ: Most words use plain Arabic ك but Persian words with /g/ may still use گ, you may need to use all combinations to find what you're looking for, different dictionaries will use different systems, Redhouse uses only ك
  • ي/ی: Some dictionaries will mix and match between Arabic ي and Persian ی, what's annoying is that when joined, Persian ی looks like Arabic ي, you may need to use both in your search. Redhouse uses Arabic ي in the middle (when joined) and Persian ی at the end (when non-joined). Arabic ى (Ya Maqsura) is rare but some dictionaries use it in Arabic words where it is appropriate like على, Redhouse uses Persian ی for this.
  • و: A couple of dictionaries will use novel forms of و with diacritics like (ۊ and ۏ) in Turkish words, most don't and Redhouse doesn't.

In the case where a quick search yields nothing, you will have to do a manual search using the pages for each letter. Spellings will vary, however most good dictionaries will have an entry to a common misspelling and lead you to the standard spelling, Redhouse does this.

• Find more here

• https://www.nisanyansozluk.com/ : The best Modern Turkish etymological dictionary. Uses only Modern Turkish and does not refer to Ottoman Turkish terms, useful for finding the etymology of a word (provides very good details sometimes) but you will have to search for the Ottoman Turkish term yourself.

• Experience is best as there is a general unwritten system for converting from Modern Turkish to Ottoman Turkish spelling when the term is not obvious, however in the case where you are stuck, see below

• https://play.google.com/store/apps/details?id=com.hayrat.imlakilavuzu : Excellent mobile app which converts Modern Turkish words to their Ottoman Turkish spelling, may sometimes miss words that are dialectal or non-standard

• https://play.google.com/store/apps/details?id=com.tevakku.ostr : Similar to above, contains more words and includes Modern Turkish translations/definitions but searching using Modern Turkish is much worse here

• For heavy vulgarizations of Arabic and Persian words (such as zerzevat, menteşe etc), it is best to try to search the vulgarization first before searching the original term to see if the vulgarization had been standardized by the dictionary, most dictionaries will refer to the vulgarizations as such and point to the correct original term.