Generically, you can click on “Issuer” on the crt.sh page. That will give you the intermediate. You can then click one of the crt.sh IDs for one of the intermediate’s certificates, and download it as above.
Generically, there can be a chain of multiple intermediates, so you may have to repeat that step.
Let’s Encryot’s intermediates are subject to change, and any ACME client should automatically download the intermediates it’s told to.
However, for your specific situation today, the fact is that all currently valid Let’s Encrypt certificates use a single one.
You can download the “Let’s Encrypt Authority X3 (IdenTrust cross-signed)” certificate here:
Or, more specifically:
Here’s where it is on crt.sh, if you’re curious: