Becoming an endorsee of a product normally comes about from a company approaching you because you are doing something in the public eye. You might be playing with a large act on a big tour, doing a tv show, have a large presence on the internet etc etc... Sometime it can come from being recommended by a friend who's already endorsing the product and thinks you should be too.
One of the main points of contact for an artist and a company besides the ones I've already mentioned is a trade show. This could be the winter NAMM show in CA, Summer NAMM in Nashville (or wherever they hold it... not quite sure...), a bass event like Bass Player Live, or an overseas trade show like Music Live in the UK or the Musik Messe in Frankfurt Germany.
I know all of this information probably won't change anything for you immediately in terms of getting some free gear from a company, but this is the reality you're looking at if that is your ultimate goal. You have to have some kind of status and public exposure to be worth something to an equipment maker, and then you can start a two way relationship that often involves getting gear for free in return for publicizing that gear through your touring, recording, or online presence.
I don't actually use any mono gear, not sure where you got that from. I was approached by TC in 2008 just before they launched their line of bass amps, and was asked to be one of the initial endorsees for the new product line. That was at Bass PLayer Live in Los Angeles, and then I cemented that relationship at the namm show in CA a few months later by performing at their booth.
I hope that info helps.