Retaining rod straps can be bought separately but they also come with some drain rod sets. The more rods you have, the longer the retaining straps will have to be.
Wonkee Donkee has found an alternative device which could be used as a retaining strap!!
Drain rod bags
Drain rod bags are useful for storing your tools and protecting from damage. Do not usually come in specific sizes, but most should be sufficiently large enough to accommodate your rods and their associated tools.
Drain rod bags can be bought separately but they should also come as part of a set when you purchase a complete set of rods.
Although there is nothing printed on a bag to indicate its size or how many rods it can hold, the average bag can accommodate up to 10 chimney rods and approx 4 drain tools depending on their size.