A cake knife doesn't need to be sharp, which means you can make your own out of just about anything. Case in point, YouTuber I Like To Make Stuff made a knife out of some scrap wood.
The blade here uses a piece of maple and the handle is made of mahogany. After making a few cuts, the rest of the process is pretty much just sanding. You'll sand down the wood to give it a knife shape, then cram the handle on with some wood glue. Everything else here is pretty much about aesthetics, but you'll find a guide to make this for yourself over on YouTube.
How to make a cake knife from scrap wood [YouTube]