You only locate Bigfoot after you have visited all four of the original sites (Loch Ness, Himalayas, Puerto Rico and New Jersey) and proved the existence of three cryptids -- Nessie, Chupacabra, and Jersey Devil. When you have gotten the footprint photo from the Himalayas, and confirmed the cryptids at the other three sites, Mews will tell you that Bigfoot has been sighted in the Pacific Northwest, and you fly there to find him and track down his home.
When you automatically inform Mews of Bigfoot's whereabouts, Gretchen Grimlock will then appear on the screen. She will tell you that the helicopter comm was bugged, and that she is planning to capture Bigfoot to make millions by charging people to see him. Mews's face then appears again, telling you to stop Grimlock from taking Bigfoot. Fly close, then jump onto Grimlock's helicopter, take off her fuel cap, and use the shears to cut the rope connecting Bigfoot's cage to the helicopter (which has a parachute). You float down to safety, then meet Mews back at his mansion to receive the million dollars.
Yes, you can find proof of The Jersey Devil, Bigfoot, Nessie, and the Chupacabra. (see related question)
first you have to find all the other cryptids then you go in the helicoptor to the new location and follow him to his cave
A gorilla
You get the Island Medallion after you prove the existence of three of the four cryptids, and rescue Bigfoot from the evil Gretchen Grimlock. (see related question)
In the Pacific Northwest, follow Bigfoot in the copter as he moves through the trees. Be careful because he may trick you. You have to follow him until he reaches the caves.
You don't. However, you will later follow Bigfoot as he travels through the trees in the Pacific Northwest.
Not Bigfoot's footprint, but possibly that of the Yeti (a similarly described creature). It is above the monastery on a trail to the upper left. You need a photo of the footprint in the snow.
He is in the Pacific Northwest, where he lives in a cave. Once you have confirmed the other 3 cryptids, you can go there and follow him in the helicopter.
just wait for it to move the follow with helicopter
You can't, because it never appears. You do find Bigfoot at the end of the quest. All you can do at the Himalayas is get the lantern from the monastery and a photo of a footprint high up on the mountain. This is not enough proof. When you prove the other three cryptids, Mews will send you after Bigfoot instead.
just keep following from a distance
After you have confirmed the other three cryptids (Nessie, Chupacabra, and the Jersey Devil), and brought back the inconclusive footprint from the Himalayas, Mews will send you in pursuit of Bigfoot, which is similar to the Yeti.