Sounds like your wheels are rusted to the axles and this is a common problem. I have not been able to find a purpose-built rear wheel puller for riding lawnmowers but I did make one using some 2" square thick wall tubing. Loaned it to someone and never saw it again. Start by getting some good penetrating oil like PB Blaster, Kroil, In-Force, etc. (not WD40) and spray both sides of the axle/wheel. Make sure to remove any wheel retainer caps so you get to where the axle & wheel meet. Spray these areas several times a day and maybe you will get lucky where the corrosion bond is broken. I know this next statement does not help you now but it may in the future: when I bought my rider, I removed the rear wheels and greased the axle shafts to prevent this very problem. Once you get your wheels off, do the same.