Are there any decent third party leather bands available?

I was looking on Amazon and a number of the leather bands got quite good reviews. Whether or not they would meet your standards is something only you could say though. Given the prices, I suspect they are not going to be quite the same quality as the Apple bands, of course.

I happened to purchase Nomad's stand for Apple Watch and I like it. I see they now offer a strap:

http://www.hellonomad.com/products/strap

(I have no financial interest in Nomad, but I am a happy customer of their Apple Watch stand.)

There is at least one report on this site of a non-Apple band that got stuck where it attaches to the Watch and couldn't be removed:

Need help getting band off watch

Be careful, because Apple won't be able to help if you have a problem with a non-Apple band.

I got clued in onto these guys... Lucrin... seem very well made, with a lot of style, leather type and color options

https://www.lucrin.com/straps-and-watches/watch-straps/apple-watch.htm