Some questionnaires or surveys are published within an article. To find them, conduct an article search in a bibliographic database on the topic of interest and add in the Keywords: survey* or questionnaire*
In some cases the actual questionnaire or survey is not published with the article, but referred to within the text. In this case look at the bibliography and find the reference to the questionnaire/survey itself, or to the original article where the instrument was published. From that information track down the instrument.
Some instruments are not free. They can be purchased from the developer.
Tips for searching for an already established survey or questionnaire. Be sure to read the definition if there is one and also look to see if there is a broader or narrower concept that might be more specific to your needs.
In Ovid MEDLINE or PubMed using MeSH terminology
Use the subheading for Statistics and Numerical Data (sn) with your topic
In conjunction with your topic use the term AND to combine the topic with any of the terms below.
There is a broader term Data Collection or a narrower term Self Report, if those are more appropriate.
There are more discipline specific subject headings or MeSH, such as the ones below. Be sure to read the definitions of these terms to make sure they are correct for what you want to find.