You wouldn't get a certificate for decryption from any public CA. Generally what you would do is generate a certificate with your internal PKI, or if you don't have one you would generate one on the firewall itself. You would then import that certificate on your endpoints so that it trusts certificates generated by the firewall.
Review the following KB https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClmyCAC that goes over the process in-detail.
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