Amazon is probably your best bet. Or like, any electronics store. The only issue you'll have is if the minivan uses more power than the mini can provide, doubt that's the case though. I think people are tending towards c because it's the new standard (and reversible). Even with led I don't think a 40% could get near max draw on a standard usb 2.
Edit random link, there's loads. The internet is wonderful.
Edit edit, as below I was wrong, micro not mini...
Amazon link with the exact ones I've got. Actually I've got so many I have them everywhere, just to be safe. https://www.amazon.com/FanTEK-Converter-Motorola-Moto-Nintendo-OnePlus-5/dp/B01N75X6MW/ref=sr_1_4?s=electronics&ie=UTF8&qid=1499887487&sr=1-4&keywords=Mini+usb+to+usb+c+adapter