How do I add DRM copy protection to my digital video downloads?

A lot of this you can create yourself if you have a website that can handle the bandwidth. HTML5 AES streaming is all pretty much built in now using encrypted media/source extensions. You can use some prebuilt methods to save time but you'll need to be a little tech savy if you don't want to buy upfront licenses and distribution platforms.

Have you looked at controlled streaming of your content?

Sell your videos online and worldwide | Vimeo On Demand

It should be noted that as your Google search found out, in this day and age it is difficult to get 100% protection against theft - so many content providers are looking at a model where the price of the content (and the convenience of access to that content) make theft too much work.

MtD

My feeling on this is that the little guy really has no viable pathway for secure digital distribution.  I think you'll find any available solution far too expensive for small scale distribution.

I should qualify that opinion by stating that I do still approach distribution from an "old school" mindset, that being living room viewing.  While many people today do watch videos on their computer, tablets and phones, I do not consider that sufficient from a distribution point of view.  The client must be able to easily get my videos onto their living room TV.  If they're also viewable on other devices, bonus.  But I would consider any solution that leaves out or makes difficult the living room viewing experience as non-viable.

Copy protection for physical DVD/BD discs is applied at the mastering stage, in the form of proprietary track structures. It's not something the editor has any involvement in - you simply pay for it as part of the production run.

Digital video-on-demand files are protected either by serving them through a negotiated and encrypted streaming service (which prevents a casual user from being able to retain the file as a playable download), or by encrypting the file and combining it with a license key that is only valid for a short time (along with a proprietary playback application that can read the embedded key). Again, these services are applied at the point of distribution by third party providers (Fairplay, PlayReady, Marlin, etc) and the editor has no involvement. Except in a few cases the master files on the server are unencrypted, and the protection is applied in real time as each customer downloads their purchase.

Adobe provides software in the form of the Marketing Cloud's Primetime system (which uses ExpressPlay from Intertrust). These solutions are generally sold to large content providers (studios and TV networks) so pricing is by negotiation only, and will be very significant. Small producers are best served by contracting with a pay-per-view distributor who handles the DRM and app development.