I have seen foot rollers with electronic keyboards carrying the hefty price tag of $500. There is even a $120,000 dollar foot roller made of solid gold. This price was before the recent rise in gold prices. The good news is the silver version is only $5000.
But fear not. There are cheaper versions. And the cheapest I know is as close as your favorite golfer.
Get a couple of old golf balls. Their condition is not a issue. Drop them in an old sock. Tie a knot and you have a pretty good foot roller. The last bit is to keep the golf balls from rolling away.
There are all kinds of found objects that work as well. Conduct your own scavenger hunt and see what you can find.
But the funniest object used by a client of mine was an old coffee can. She loved to roll her feet on it at work. This worked well until the coffee can shot out from under her desk and came to a stop in front of her boss. He was a bit puzzled and she was more than a little embarrassed.