So, the best thing I've found for that is the below. It doesn't stretch the entire top at once, but if there's a part above your toes that is too tight, this will stretch it inch by inch. (It's also great for shoes that pinch your pinky toes on the sides.)
I've also tried these, but they don't really work how you'd want them to. They make the shoe entrance higher, maybe, but won't stretch out a toebox.
It depends on how big your bunion is and the material of the shoe you’re stretching. More than anything you want shoe stretching liquid to put on the area. Use it when you’re wearing the shoes too to continue the stretching. If the bunion is small, use a shoe stretcher that looks like a shoe tree but with the metal oval in the bunion area. If it’s really big, you’ll want to invest in a stretcher that looks like this: FootFitter Cast Iron Ball & Ring Bunion Shoe Stretcher Spot Stretching Device
Good luck!