In case you're unaware, the 1940 census has been released and is available online for free.
Unlike the older U.S. Federal Census images available online, 1940 was not searchable (since it was not available to the public it couldn't be indexed until it was released). Indexing began as soon as it was released but because it is available on so many different websites, it's hard to know if the state you're interested in has been indexed. "The Genealogy Insider," the blog for Family Tree Magazine has posted an update listing information, including which states are indexed, at each of the different websites which offer images of the 1940 census. Check it out here, http://blog.familytreemagazine.com/insider/2012/05/01/1940CensusRecordsAndIndexesUpdate.aspx and check back periodically for new updates.
The indexed states are:
If you know the address where your person of interest lived in 1940 (or think you know) you can browse the unindexed images by following the directions given on the site of your choice.
If you want the index to be completed faster, volunteer to help index at the 1940 Census Community Project, https://the1940census.com/.