You can create a "virtual corpus" by selecting texts from among the more than
22,000,000 texts in the iWeb corpus.
Note that via this search form, you can only enter one single word in the [WORD] field. If you want to do more complicated searches
with multi-word strings, substrings, lemmas, grammatical constructions, etc then you should create a virtual corpus using keywords. See the
instructions by clicking on TEXTS/VIRTUAL in the search form, and then "Keywords in the Text" (the first link in that help file).
After you've created the corpus, you can add or delete
texts and make other modifications to the corpus. You can also search just within the virtual corpus that you've created;
you can compare the frequency of words and phrases across your different
virtual corpora; and you can generate keyword lists from any of the virtual corpora.