Some of the most well-respected polls for US elections include:
RealClear Politics
FiveThirtyEight
Ipsos
Siena College Research Institute
These sites conduct their own polling. Ipsos and Siena in particular often work with news organizations like Reuters or the New York Times to cover election stories.
Voters should look for polls like these that pull from a representative sample of Americans. Polls conducted by cable news shows are inaccurate because their audiences can skew in one political direction. News organizations can work together with polling companies to offer robust polls, but polls that news organizations conduct themselves should be treated with caution.