The freelance market. The submission pages of print and online publications. The normal exercise for most sets of eyes reviewing these things are looking at gig opportunities that might fit.
Those opportunities can transpose themselves into growing your portfolio of potentially sell-able content. A particular listing may be topic specific to something that interests you and therefore an easy assignment. If the freelancer does not have enough time to make the deadline, the assignment might be enough of a spark to still inspire quality work to pitch and sell another day.
Another potential client may seek similar content down the road. In addition, what could possibly stop you from sending a pitch to the original ad poster in a few days or weeks time? There is always the chance they may not have purchased the content they need. So another opportunity may open for the future.
Everything can be a prompt. Use the market listings to the best possible advantage. Everything starts as something. So start somewhere.