There are thousands upon thousands of different fonts out there. Picking which font to use may seem trivial, but the look and functionality of the font(s) you choose will determine the tone of your greater design. While many of the software tools used to create research posters come with several dozen fonts to choose from, sometimes you want a different look. Here are some resources for finding free fonts online:
Open Foundry is a free platform for curated open-source typefaces.
1001 Fonts is an impressive repository of both free and paid fonts for use in design.
dafont.com is another online warehouse of fonts. Lots to choose from, easy to navigate.