I am assuming, like many people who are not familiar with booking travel online, that you don't know how to search for business or first class airfares on airline and other websites. Many people here are so used to that they forget it is sometimes not very intuitive.

All the airline booking sites have an advanced flight search page, usually accessible by a tiny little link below the space where you enter quick search details on the main airline page. If you look, there should be a box labelled class of service or cabin. that is where you select if you are looking for coach or economy fares.

Keep in mind that from Denver, only three airlines offer direct service. And two of them - JetBlue (B6) and Southwest (WN) do not have first class cabins on board their planes.

You should also do a search on this forum for how to use ITA Matrix. This search engine offers a lot of advanced search features and ofter turns up some good deals, especially if you are willing to make a connection.

I would not get my hopes up on finding something cheap for this Mothers Day. You are too close to departure date for the best airfares, and Sundays are especially pricey.