The price of an inflator can vary a lot. Its power, its voltage, its efficiency, even peripherals such as its hose and nozzles, all influence the price. So, you can find cheaper models starting at £ 45, and the most expensive ones at around £ 450.
Because of the variety of products that "serves", the inflator does not have a specific niche. Thus, you can find it in various types of physical shop. Still, construction and housewares shops, such as Leroy Merlin or Americanas, are the safest bets to seek varied and quality models.
Now, on the Internet you can easily find a wide range of models and prices, especially if you browse through sites such as Amazon, Mercado Livre, which, to top it off, allow you to make the purchase from the comfort of your home.