How long of an advertisement? If you're talking about short ads like AdWords and other text ads, visit SpyFu.com and enter the keywords for your product or service. You'll see who the top advertisers are on AdWords for those keywords, including what their recent ads are. Obviously, you wouldn't want to copy them exactly, but it will help stir your creativity.
For long-form ads, good copywriting is a skill and an art. It's hard to produce anything off of a sample or template, that actually has a chance of converting readers into buyers, without either acquiring or hiring the skills needed to write a good sales letter.
Frank