找头 is colloquial; in more formal contexts you'll see 零钱 ('small change') or just the verb 找 ('to give change'). The 头 here is a noun-forming suffix, not 'head' - like in 念头 (idea) or 苗头 (sign).
memory
Think of 找 'searching/finding' + 头 (suffix) - what the cashier 'finds' in the till to give back to you.
