nobody else seems to have mentioned,
if you own every machine that connects to your site
eg "this is probably not what you want"
, like a corporate setting, you can create a certificate authority of your own, install it's public cert into all the machines (well onto all the browsers and cert stores) that connect to your site. this option is free of third party monetization; it's the same encryption cipher, you don't get signed into the public trust though (eg your authority is not recognised by Google's Chrome, Mozilla's Firefox etc, the way Let's Encrypts's does) but it will be recognised by machines you configured to trust yourself.
admittedly the deals off setting up a certificate authority are tricky somewhat obscure and maintenance can be a lot of work- so I'll leave them off here, you're probably best doing a deep dive research on the topic of you're truly interested in this approach.
though for a nieve deployment, if you can try XCA
XCS is is a decent way to issue certs for tiny deployments and includes help documentation that walks through the whole setup.