You can buy patching plaster in fairly small bags at a paint store. Not sure if a box store will carry it. Even a large bag will not be expensive. You should plan on a thick mix to fill in the space and then a skim coat layer to surface to the wall. Use a nice wide drywall knife or plaster trowel.
You could probably get away with a hot mud drywall patch too but it will heat and cool and therefore expand and shrink at a different rate than plaster.
I mix either for patches right in my drywall pan.
Make sure you remove any plaster around the hole that is not firmly attached to the lathe even if it means accomplishing a larger patch.
Be sure and let the plaster or drywall patch completely dry and cure before priming and painting it.
The Following User Says Thank You to user1007 For This Useful Post:
I've never used patching plaster, so I can't speak for that. I would cut a piece of drywall to fit the hole (as close to the shape of the hole as you can) and screw it to the lathe. You could shim it to get it as close as possible to the correct thickness. Then use mesh tape and setting type compound to finish. The final skim coat can be regular ready-mix. Just another option.....
If you can read this, thank a teacher. If you can read this in English, thank a soldier. Support our troops.