There is no 'true' coin, ancient coinage varies enormously by region and era for all sorts of reasons. Although the term 'Charon's obel' might imply there is some kind of uniform coin, the evidence does not support a standard "I am dead, 1 entry ticket to underworld pls" coin; the coinage used (if at all) appears to be non-standardised and varies wildly.

The selection of a denomination [for burial] is probably the most frequently noted criterion. R. Delmaire insists that the Greeks speak of an “obole for Charon”. It is therefore a small silver coin one-sixth of a drachma whose mass varies according to the standard (Nicolet-Pierre 2002, pp. 138 and 140). The Aeginetic obole weighs 1.05 g, the Attic one 0.72 g. But in reality, the gravess of the second half of the 4th c. bc show significantly more bronze than silver coins, for example at Metapontum or Heraclea Lucania (Parente 1999, p. 142). (Jayen, pp 11-12.)

Sources and further reading: [Jayen, J.M. 'Charon’s obol » : some methodological reflexions'. The Journal of Archæological Numismatics 2012/2, p. 1-18.] See esp. pp 11-12.

Two things:

• The coin that Greeks buried with their dead wasn't a drachma, it was an obolos. (A coin of less value than the drachma, which is derived from Δρασσομαι, the Ancient Greek for "to grasp", because one drachma was roughly a handful of oboloi.)

• As for the design of the obolos, since various city states all had their own mints, designs varied. The only things oboloi from different city states could have had in common was material (usually silver) and shape.

I'm not sure what you want the tattoo to look like, but a quick search could get you a few pictures of recovered ancient oboloi. If you want an inscription to point out that the coin is an obolos, something like ΧΑΡΩΝΙ (It means "For Charon", as Charon was the ferryman who supposedly took sould across the Styx) would probably suffice.