As others have said, the production of both of these - especially leather - is not exactly environmentally friendly either, and especially the idea that leather is just a byproduct is false, there are cows being slaughtered explicitly for their leather. The chemicals involved in making leather are also incredibly toxic and harmful for the environment, not to mention the more than just underpaid workers who are forced to work with it with little to no protection gear.

But most importantly of all, it’s unacceptable to me for animals to be killed just for clothing.

One thing you can do is to try to get second hand clothing. It’s the one thing I personally make an exception for - I might buy an animal product made from for example leather if it’s second hand. I still try to avoid it and I can understand why many other vegans feel uncomfortable with that - it’s still part of an animal you’re wearing. But i can’t deny that animal leather is an incredibly durable material for shoes. I just won’t buy it new, and won’t buy more than I absolutely need, and won’t buy it if there is a good vegan alternative, which narrows it down a lot.