Melvin,
If you want to be an endorsee, make some GREAT products and get some famous people to like them
I think you mean, endorser. The endorsee is the one getting the endorsement (making the product); the endorser is the one giving the endorsement (using the product).
IME/IMHO, companies get annoyed when people email them to ask how they can "get" an endorsement. You don't get one; you give one. If your name carries any weight, your endorsement of their product will mean something, and they might encourage you to continue using their products in public by offering a discount, free gear, etc, as a win-win thing.
If you see somebody say "get an endorsement," they probably meant to say "an endorsement deal," because that's what they're getting. It's kind of backwards, I know.
Anyway, as far as endorsing a product, companies are usually happy to let you go ahead and say all the nice things you want about them. Go for it! They're not going to stop you. However, if you want something in return, you're going to have to show them that whatever they're giving you is worth at least the same (if not less) than what you're giving them in return. Usually, they will want to see a lot more than a demo. They will want to see a press kit, complete with a list of people you've toured with, clinics you've done, video and radio play, internet site traffic, etc - and some evidence that you are known and admired as an virtuoso, and someone with identifiable and enviable tone. If you're not signed to a least a mid-level label, there's very little chance of getting an endorsement deal that's worth anything, especially for a bass player (it's a lot easier to get a deal for guitar picks, strings, etc, and it's doubly easier if you're a guitar player and not a bassist or drummer... The public tends not to care what strings the bass player in a group uses, but everybody recognizes "Gibson" or "Fender" on the guitar player's headstock).
You can endorse all sorts of things. Just poke around. Offer a testimonial to a company that makes stickers, for example, in exchange for extra stickers. It's easy for them to give you free stickers in exchange for a quote from a satisfied band on their website - doesn't quite work that way with thtings like instruments.
Hope this helps,
- Dave