You can get parts from any hardware store. They have them at Walmart. You can order from Amazon too. So lots of options. Here’s an Amazon link:
https://www.amazon.com/Fluidmaster-400CRP14-Universal-Flapper-Toilets/dp/B007TUHQWY
It’s cheaper at Walmart by a couple dollars. You can shop around by searching for “toilet tank repair kit.”
The kits come with fairly good instructions on changing out all the parts. If you need something a little more in depth or like watching videos check out YouTube. Here’s a video link for the kit in the Amazon link:
You can always come back here for help too! Good luck.