hmmm

This depends on a number of factors.
If you are dealing directly with the client. Ask for their university's online library access. If the online library can not be accessed remotely, ask your client to search and send you the needed information.

You could by Questia. Its membership is for 100USD a year (the last time I checked).
There is NetLibrary. It is paid.
if you are into this business then it is good to invest some on getting access to libraries.

You could use google books. (free)
And there are plenty of sources you could find for free such as online articles at b-net, websites of specific organizations (companies, government, NGOs).

I use questia and its good. I usually find about 10% of the sources there. If I use 8-10 sources in a paper, it is very likely that one of them would be from questia.

50% sources would come from Google.
I also ask my clients for access to their university's online library.
Text books and Google help a lot.
I recommend keeping a good text book of the relevant subject as well.

Happy Writing

was it useful? :S