I generally grab my own screencaps to create artwork, but occassionally I do use online resources therefore I have listed below (in alphabetical order) the sites that I use within the corresponding fandom:

Starfox's Mansion (The Sentinel)

The Sentinel Episode Archives

Screencaped (Multi-Fandom: Almost Human, Teen Wolf + many more)

Grande Caps (Multi-Fandom: Almost Human + more)

Home of Nutty (Various TV shows and Movies)

Midnight Road (Almost Human)

Gallicka (Multi-Fandom: Merlin + NCIS, Hawaii 5.0 + many more)

Screencaps At Aithine (Multi-Fandom: Hawaii 5.0 & Sherlock)

DJ Capslock (Multi-Fandom: Too Many To List)

Swannee's Screencaps (Multi-Fandom: Castle/Hawaii 5.0 + more)

TNG Trekcore (StarTrek The Next Generation)

Krissie's Caps (Multi-Fandom: Warehouse13, Haven + more)