Dryer Balls Questions

[Goldcountry-cloth]

I am looking into getting some dryer balls. We have a really old dryer, and it takes 70-90 minutes to dry a regular load of laundry. I have heard that some dryer balls leave stains on clothes? They seem kind of pricey, where do you buy them? Do they actually save time and money? I wouldn't be using them for diapers because we dry outside. Thank you!