Here are a few online resource recommendations.
There are many others out there as well, such as Life Print, but Signing Savvy and Hand Speak use video clips rather than images which really helps give a clear demonstration. If you prefer the images, go for it! I personally find the video clips clearer and more helpful. I typically Google whatever word is needed and then look for either of these to come up (the search on the sites themselves sometimes give me trouble).
YouTube can also be a good source. It can just be difficult sometimes to know the credibility of those who made the video or simply that they are in the same region – yes, region matters even in ASL; one sign here can mean something different in another part of the country.
I am sure there are also a plethora of apps out there! If you know of any, please feel free to send them my way. I have tried a few and have not liked them but there are always new ones popping up!