Online newspapers, online advertising sites, and sites with products related to your focus offer ways to leave text and graphic ads for a specified length of time.
Newsletters and ezines are more fleeting but used repetitively may result in frequent exposure of your product or service.
Ad sizes start with a few words or single line and theoretically have no limit. There are many resources offering free ads. You might get larger ads, ads that run more frequently or targeted exposure by paying for advertising.