I'm sure both Kirby and Aerus want their customers to come into their respective stores to purchase accessories like bags, filters, belts, etc. They can of course charge their hefty prices.
However, I always feel when you go into one of those specialty stores, you know that for the most part, you are going to get the right part as the sales people will know your machine. Try going into Walmart and ask one of their reps if they have X-bag to fit your Kirby or Aerus machine. It's likely they may not even know those are the names of vacuum cleaners. They are probably used to hearing the name shark or whatever other bagless stuff they have on the shelf that week.