If you search on "timestamp", "timestamping", and "notary" on Crypto.SE and Security.SE, you'll find lots of references. I've collected a number of timestamping services that were mentioned in one of those places; this should provide a number of companies and online services you can check out:
I cannot vouch for or endorse any of those.
See, for instance, the following questions on this site, and the following other resources:
Signature and Timestamp for Long Term Document Archival Question
Timestamping services authorities, utilizing Linked, hybrid and other than PKI schemes
Happy reading!