Hm, that's pretty weird they wouldn't just exchange the pack of coils for you. But I guess each store has their own policies. I mean..I guess if you are in the market for another tank maybe buy the one that fits those coils?
Other than that, try
E-cig Marketplace | Or the sale/trade page on ECF
You might be able to get some of what you paid for them back, however I wouldn't expect to get your entire cost back, especially factoring in shipping if you can't get the buyer to pay for it. Don't beat yourself up over this to much...you expected the VAPE STORE employee to have knowledge on VAPING GEAR...I would be ...... too lol.
Did you try talking with the owner of the vape store? Maybe send an email just stating what happened and asking if there is anything they can do for your situation? If you did and nothing came about from that either, then i'm not sure if there is anything you can do sadly. Time to throw another 15$ to coils that fit. I really hope you aren't being sincere when you say that 15$ makes you too stressed to continue btw.