There is a harder gum called "Falim gum", but this gum suffers from the problem that most gums do; it is made out of ingredients that usually contain varying degrees of substances you do not want in your body. Even though a person obviously never swallows gum while chewing, it is unavoidable that traces of it will make it into your body as you chew and swallow every now and then.

For the reason mentioned above I recommend the following gum; https://www.amazon.com/Xylichew-Naturally-Sugar-Free-Chewing-Spearmint/dp/B00IJVDIR4

This is the gum that I intend to buy. Thus far I have been relying on a big ball of gum from regular sugar free gum that I bought in my local grocery store. I take 10-20 of those small pieces of gum and chew it into a big ball of gum to increase the resistance. It gives me quite a good chewing workout!

Just make sure to go easy in the start, make sure to not develop any TMJ issues.