I had the same problem finding art to fit exactly the IKEA frames I'd bought when I discovered this Ebay shop:
items in Art For Ikea Frames store on eBay!
Each of its listings stocks some really great photos that they re-size on glossy professional photographic paper to exactly fit any of the most popular RIBBA Frames.
I also had some of my own amateur photography to display and contacted them for a quote which was very reasonable. They printed my photography that they'd re-sized to fit exactly within the IKEA provided mounts so it looks fantastic.
Getting custom framing would've been SO much more expensive.
The results were fantastic, highly recommended.