I'd like to put up some posters around where I go to school, where would I find some great looking posters, flyers and tips on advocacy?
You can find lots of marketing materials on Spread Ubuntu, they have some great looking SVG posters which can be downloaded and edited using Inkscape. You may also be interested in joining the Ubuntu Marketing Team or the Ubuntu Art Team which are both involved in created posters and logos.
You can also find some nice materials on openclipart, while not Ubuntu related they do have svgs which are useful for making materials.
There are some good designs available in the Ubuntu deviantArt group, but be careful of the licensing which is shown on each work individually.
Don't forget to upload your own results after editing to share with others. If it's Ubuntu specific, then spread ubuntu is a great place to upload to.
An important note to consider when making Ubuntu posters is the licensing. Make sure to only get your source materials as either public domain (such as openclipart) or free creative commons licenses such as CC-BY and CC-BY-SA. This will guarantee that you can continue to collaborate and share your work with others without restrictions.
The LoCoTeam might also have info about localised poster & flyer designs, and other promotional material. Or they might want to cooperate with you in having something printed (printing in larger volumes is a lot cheaper).
In addition to the above, you can also organize a Software Freedom Day event. Some of the flyers / posters / etc. are generic enough to be used year-round. You can find some of those at: