How did early europeans obtaintheir supply of slaves?

The same as any member of a dominant empire - took them from civilisations seen as "lesser". The earliest known large European civilisations, the Greeks and Romans, took slaves from other European and Middle Eastern civilisations. Muslim Europe took slaves from Africa and Asia. Mongol Europe didn't tend to enslave but instead either to massacre and include the remaining vanquished and allow them a share of the next victim's wealth. Northern Europe under the Vikings or Rus didn't tend to enslave either. Christian Europe post renaissance mainly plundered Africa for slaves.