Understand your problem and don't think you'll find a commercial one. We have one panel nearest the house which is needs to be 6' high at the start, dropping to 3' high to match the rest of the fence. Our fencing contractor took off the top beading strip, sawed it down at an angle of 45 degrees to make it lower gradually like a ski slope and then nailed the top beading strip back on. This looked fairly easy to do with an efficient electrical saw. If you want it curved it will obviously be a problem because your husband will have to do it himself and it's virtually impossible to find a curved top beading strip to put back on the top of the fence panel afterwards to protect it.
I take it you're using the standard purchased 6' lap fencing panels available from fencing suppliers. If you're building fencing where one vertical stip is nailed on at a time (is this called lap boarded fencing ?) then an electric saw might be able to do it if you chalk out the curve you want first, but if the saw slips and you get it wrong, you've wasted a lot of wood and time and effort!