Game marketing

Are there companies that help indy developers promote their games for either a fixed amount or a share of the income?

These are usually a bad deal.

Far too many bad companies make deals like that where they have few concrete obligations. They can satisfy the terms of the agreement by including the game on a web page along with thousands of other pages, and bots clicking the links or search engines indexing it are enough for the minimum metrics. Technically they promoted it, and technically the page was viewed a number of times, even though it was entirely bots and spiders. If the game becomes successful due to the work of the developer, even if the so-called publisher did nothing, they can still demand the contractual large percentage of the revenue.

Consider it as a business to business contract. You are hiring the company to do specific things. The more specific the list of what the company will do for marketing, the better the agreement. You should have a long list of what services you want and what investment is needed, not vague promises or metrics that aren't actually meaningful to success. Too often they bind the developer and don't specify anything meaningful from the bad publisher.

Also have a lawyer who knows the industry, or is a business and contract lawyer at least familiar with the industry, review the agreement before signing.

There are good publishers out there, but they are not hunting down indy and hobby developers, instead they have a lot of bids and offers and solicitations that they are turning down because they too have constraints on resources.