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:

- http://www.proofofexistence.com/

- https://www.btproof.com/

- https://www.universign.eu/en/

- http://www.guardtime.com/

- http://securo.it/

- http://www.cryptostamp.net/

- http://www.itconsult.co.uk/stamper.htm

- https://www.globalsign.com/timestamp-service/

I cannot vouch for or endorse any of those.

See, for instance, the following questions on this site, and the following other resources:

- http://security.stackexchange.com/q/19184/971

- http://security.stackexchange.com/q/43995/971

- http://crypto.stackexchange.com/q/1816/351

- http://crypto.stackexchange.com/q/3291/351

- http://crypto.stackexchange.com/q/3733/351

- http://security.stackexchange.com/q/24458/971

- https://en.wikipedia.org/wiki/Trusted_timestamping

- http://tools.ietf.org/html/rfc3161

- http://security.stackexchange.com/q/32493/971

- http://security.stackexchange.com/q/47289/971

Happy reading!