Which tokens can I use in a rule to email all suppliers mentioned in an entity reference field?
Assuming your question is about D7, you could try to create a view (using the Views module) of those suppliers that you want to send an eMail to.

After you succeeded in creating such view, you can process the results of a VBO enabled view via Rules, whereas your rule performs 2 Rules Actions (in the rule you already have in place related to your Rules Event about "completing the checkout process":


load the VBO list of those suppliers.
add a loop to perform a Rules Action "send an eMail", for each supplier contained in your Views results.


For more details, have a look at the tutorial "How to process the results of a VBO enabled view via Rules?".