Where do scientists obtain primers to be used in pcr and in this technique answer?

In order to happen DNA synthesis of a wanted portion of DNA in laboratory, the DNA polymerases that preform that synthesis need to have a an existing strand of nucleotides already in order to attach new DNA nucleotides. To that existing strand of nucleotides we call primer. In a lab, scientists synthesize DNA primers through specific sequences that bind to other sequences of a single-thread DNA molecule and these are the primers commonly used to perform PCR.

Idont really understand