There are companies that compile lists of people interested in information on various subjects and what sort of information they would like. Basically mailing list what you do is buy a list from one of these companies and send your emails to them. However you need to realize that you may buy a mailing list that may not be specifically targeted to your niche.
One of the popular ways of building a list today is to do list swaps with marketers in the same niche or with products and services that compliment your products. An extension of this idea would be to form a sort of partnership with these marketers mailing list and jointly buy lists in the same niche thereby sharing the costs. Treat them as if they were an important customer. You should be able to offer a service that is uniquely beneficial to you marketing partner Offer your client what he cannot get on his own.
Is your joint venture partner a big player that is almost neglecting the customers at the base level of the industry? Enable him to feel the mailing list gaps with your links. Is your player concentrating on the main features of his/her service offerings? Link the minor features to yours and provide a hybrid that is appealing to your partner.