There's quite a few different levels of endorsement - the most basic is just saying you use and endorse a certain brand of bass/amp/shoes/deodorant or whatever. That's fine, but there's nothing in it for you.
The next is where that is reciprocated, where the company in question gives you a mention on their website, or in a list of artists who use their stuff, but nothing else. This type of endorsement is fairly easy to get, especially with smaller businesses, such as custom luthiers, and it is a little extra publicity for you.
Next up from there is the "discount" endorsement - similar to the last one, but where you are entitled to a discount on the company's products. This will almost entirely be based on enquiries the company may have received because of your endorsement. It might be 5% off, or 50% or more off. This is probably the most common form of commercial endorsement deal.
Next up from that one, is the "free, as in beer" endorsement. Here, the company in question will give you free stuff, usually with certain stipulations, such as using their gear exclusively, making sure they get mentioned in all promotional material, and being able to use you, your image and the name of your band in any of their promotional material. The deal might also include promotional tours. You'll usually only get to this level if you have a high national and international profile, where the company can be sure of additional sales because of your profile.
Then, the gold standard - the "signature" endorsement, where the company in question will work with you to design a "signature" product (instrument, amp, shoe, etc). This will almost certainly require promotional touring, appearances at trade shows, etc, and is usually reserved only for those with very high profiles.
In all instances, you have to make yourself and your band an attractive business proposition for the company in question, if they are going to have a "business partnership" with you - because, basically, that is what it is. You'll need to send them your press kit, a list of achievements, reviews, etc, and make it clear you actually like and use their stuff - if not, it may come off rather mercenary.
Start small - a custom luthier, custom pedal maker or one of the smaller string companies would be a good first bet. Not much chance for free stuff, but definitely good from an exposure standpoint, for both of you. Once you get up a few notches (maybe up to the indie deal and local touring level), major companies like Peavey, Carvin and ESP may start showing interest in punting you discounted stuff. This is always a good move to make, especially with the likes of Peavey and Carvin, since they can usually arrange to hook you up with amps, drumkits, etc in different places that you may tour, easing your burden.
This is all just from my personal experience - YMMV, of course.

Hope it helps.