Could you please tell me where Cain got his wife and where I can find this information in The Bible?   If Cain and Abel were Adam’s and Eve’s first children, were there people prior to Adam?

From God’s Word, we can only know where Cain DID NOT get his wife. Adam, Eve, Cain, and Abel, were the only ones written living in ‘Eden’ at the time, meaning ‘paradise’, or called the Garden of God, in ‘the presence of the LORD’.

Gen 4:16     And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden.
17 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch. (KJV)      

     Just before this verse of Gen.4:16, Cain was near the ‘presence of the LORD’, along with Adam and Eve. It’s important to understand that wherever God is, that’s where His Heaven is. Heaven was established upon earth during the time of Adam and Eve, just before the fall. That’s the meaning of “from the presence of the LORD”. Our Father was there, as He will bring Eden (Heaven) back to His earth in His Eternity (see Rev.20, 21, 22, and last verse of Ezekiel). The ref. in Rev.22 to the Tree of Life (Jesus Christ) locks this in with Gen.2-3 also (note the Cherubims of Gen.3:24 guarding Eden, i.e. Heaven, where Christ also dwelt, called in Genesis the Tree of Life). So as Adam and Eve were to be the specific bloodline which Christ would be born from, (Gen.3:15; Luke 3), then no others were setup there in Eden. Then Gen.3:24 says Adam and Eve were booted out of Eden, specifically where the Tree of Life (Christ) was. This is symbolic of God moving His Heaven (Eden) off the earth, into the dimension it is in today, with a separation taught in Luke 16 by our Lord and Savior.
Even though some teach that there were other daughters of Adam at that time of Cain, the reference they use is not written in God’s Word until ‘after’ Cain was put away, out of Eden. Adam had other ‘sons and daughters’, as written in Gen.5:4. Thus, Cain more than likely took his wife from the ‘land of Nod’, away from God’s presence, ‘east of Eden’, as written.
I recommend a work called Sargon The Magnificent, which documents from the Assyrian Babylonian Tablets that ‘Sargon’ was a Semite, who migrated to ancient Sumer (Babylonia), and brought knowledge of agriculture and science. In the Tablets, this Sargon is named the ‘son of the dragon’, or son of bel. And later, other Semites moved there also. The city named in those Tablets of his time is Urech, which could be the city of Enoch which Cain built.
Yes, there were ‘other’ peoples on the earth during Adam and Eve’s time, ‘outside’ Eden. As God said Zion is where He has chosen for His dwelling, and ‘the land of Nod’ is ‘east of Eden’, then God’s Garden of Eden was where Jerusalem is today, and ancient Babylon is ‘east’ of Zion also. This is why many scholars assign the man Cain as being oriental, but he was Semitic.
My interpretation of Gen.1:26-27 is that God created all ‘races’ of mankind, and the specific eth’Ha’adham (this same man Adam) in His Garden of Eden, all on the ‘sixth day’ of flesh creation, but they had separate dwellings. The Hebrew word for ‘man’ in Gen.1:27 is ‘ha’adham’, meang ‘the man Adam’. The Hebrew word for ‘man’ in Gen.1:26 is ‘adam’, with no article, meaning ‘mankind in general’, i.e. the various races. The geneaology from the man Adam to Christ is given in Luke 3.
Cain is not in Adam’s genealogy given there. Cain was not Adam’s son. See my Genesis studies for more.