I FINALLY figured out how to get a 'spacer'! Since 6v vaping takes two RCR123A 3 v batteries, and 3.7 v vaping takes 1 3.7v battery and a 'spacer', I need a 'spacer' that's the same length as the 3.0 v or 3.7 v battery (RCR123A or 16340 battery).
So I searched for 'battery spacer' on Battery Junction and found a CR123A spacer for around $2. Order placed, should be here Monday. (I got some extra 1640 batteries while was at it - I only have 4 of them now!)