Homebrew is your best, and most fun, bet. Make up something, run it by the DM and give it a whirl. Tweak as necessary for balance. I've made thousands of new spells between 1979 and the present. I admit that the first spell that I made - a first level spell that gave my wizard the ability to kill dragons with a touch, wasn't my best work, but I was 6 years old.
Also, there were dozens of supplements in the AD&D, 2E and 3E eras that were filled with spells. You can often find them in used book stores very cheaply. They can provide a lot of inspiration for new spells for your world.
In my game world, I try to have the players use the PHB as a starting block, but allow them to suggest new spells freely. Also, NPCs usually take less than half of their spells from the PHB. I give them new and exciting magic to keep it from feeling like making magic choices is like selecting off the McDonald's menu and more like cooking in your own kitchen.