I have/had a bunch of Gigabyte 750Ti/970/1070/1080Ti cards. The fans on the 970s died within a year and the fans on the 1070s died within 6 months. Meanwhile, I have 750 Ti's with mostly working fans.

All these cards have crappy sleeve bearing fans. Except for the AORUS series, which is the flavor I have my 1080 Ti's in. The AORUS series have double ball bearing fans which are basically what every card should have; they're superior.

I have also contacted Gigabyte, trying to buy replacement stock fans but they do not sell any parts. But they replace it under warranty, but that might take weeks/months depending on your location.
So you can really only get replacement fans on ebay but they will die just as soon as your stock fans did. But if you send the card back for a warranty process and if they notice that you have replaced the fans, your wannanty will be void so in case the card dies, you have to put the original fans back.

In case you're going with ebay fans, you really only should look at how much amps the stock fans pull after checking yourself (for example 970 G1 - 0.20 amps, 1070 G1: 0.35 amps) and use fans that's equal or lower than that value (unless you want to power the fans straight from your PSU or a fan header). Some cards Gigabyte cards have different fans with different amps so better to check all three and most importantly, check the headers and only order what you can plug in without having to cut cables and replace headers.

TL;DR: only buy cards with double ball bearing fans or you're going to have a bad time.