Carbon clarification

[HAYLEYSOO]

Hi -- we played for the first time the other day with four people and ended up in a situation where we ran out of carbon pieces. We looked at the rules but couldn't find anything on what to do in that scenario; can players only gain as much carbon as there are pieces for (which makes the most sense, we think), or could you use additional "tokens"/placeholders to represent additional carbon pieces until some of them are freed up?