Mattermost can be categorized under Team Chat Softwares, which is commonly listed under Communication tools.

Not all similar tools offer the same features. Most include a free plan or trial, so you should be able to give them a try. You can browse some of the alternatives to Mattermost listed in the directory.

Some tools might also offer discounts or offers, especially for newer members. Most of these offers traditionally come as part of affiliate programs or promotions, but often you can reach out to companies and check whether they can give you special pricing.