how to search words / clauses in classical / literary chinese?
to search words / clauses (combinations of 2-9 characters) in classical / literary chinese, it's straightforward; if you know the methods, and able to visit the recommended sites {some of them are blocked in that "area" :( }

in short:

1   using traditional chinese is a must, no simplified chinese

2   knowledge of variant (異體字)

3   search by google

4   using reliable, trustable databases

5   able to read classical / literary chinese

6   knowledge of cantonese is a plus, a big one

in details:

using traditional chinese is a must, no simplified chinese

as i know, databases containing texts of classical /  literary chinese are all in traditional chinese. and, the imperial collection of four (四庫全書) are written in traditional chinese. meanwhile, simplified chinese was introduce in 1950s, in that area.

occasionally, some modern chinese words has a long history, way back to thousand of years old. so, if one want to know "did" any particular words appear in literatures of yore, the search must be in traditional chinese. 

eg  "泊车" ---> "泊車"

knowledge of variant (異體字)

i would recommend these two sites, for checking variant character:

教育部異體字字典

國際電腦漢字及異體字知識庫

sometime, particular character would be used in ancient time, that, another one is used nowadays. so, if search cannot trigger desirable results, use the variant.

eg  並 (u+4e26) --->　竝 (u+7add)

based on my reading experience of numerous books of 四庫全書, 竝 (u+7add) was "trendy" in ancient time, that 並 (u+4e26) is rarely used. so, if "並" do not trigger any result, use "竝"

eg  "沙上並禽池上暝" ---> "沙上竝禽池上暝"

search by google

there're significant differences in searching, by google or bing (no noticeable difference in mac or windows)

sometime, there're hundreds, or thousands of results, my trick is:


  "searchword" -china -.cn -baidu -baike


only using reliable, trustable database 

now, this's the core of this answer. remember "garbage in, garbage out"

as at june 2017, recommended sites for searching in literatures of yore are:

Kanseki Repository

this database was opened to public in march 2016, the best in current time. there're minor caveats, for average usage, it scores 90 out of 100.

漢文大藏經

buddhism has a large influence in chinese culture, this one is important.

internet archive

the internet archive has full-text-search function now, searching words in traditional chinese would trigger interesting results :)

chinese text project

this one is, 50 of 100; worth a try before "surrender" (rant: certain ocr scannings are "full of errors") 

the only two "dictionaries" that i trust:

國語辭典

粵語審音配詞字庫

personally, i do not believe any sources in simplified chinese; i might read, but, i always counter-check with other reliable sources.

reminder: garbage in, garbage out

able to read classical / literary chinese

though chinese texts are punctuated nowadays, text in literatures of yore are not. so any search results must be read carefully. do the 句讀 independently.

sometime, combination of characters might had other meaning, or, these characters are put together "by chance".

knowledge of cantonese is a plus, a big one

well, cantonese is highly correlated to middle chinese, some terms in ancient time is still used in cantonese today. further, if phonology is touched, cantonese has significant advantages over . . .

stop here now, have fun