There are many options avaialble. Generally you need to be a big player with big money for the top engines to talk paid inclusion with you. Some of the niche engines may be a better ROI if they match your target market. Also don't just think about the engines. Think about offering to sponsor some on-topic non-commerical sites in exchange for links to you. A good boost of inbound links will get you indexed and more importanly ranking in the engines.