Closely related: Can I lookup a PNR without going through the relevant airline's website?
Your travel agent should be able to provide you with your booking reference / record locator / passenger name record (PNR), and is probably the easiest to get a hold of.
Note that Aegean also allows you to check in online using your ticket number or using a tour operator booking reference, if your travel agent has provided you with either of those.
If you have a Miles + Bonus account and that number was submitted with your reservation, you may try logging in to that account to see if it is there.
If not, it is a straightforward matter to call the airline directly and ask; the agent will likely ask you some questions to confirm your identity first. It is extremely common for them to look up reservations using your name, travel dates, and itinerary (points of origin and destination and/or flight numbers).