Vistaprint do them at a very reasonable price and I’m sure a Google search will provide many other suitable suppliers. In the UK you can get one car door magnet (A4 size) for free if you’re happy with the designs they suggest or you can upload a picture and fully customise your magnet for £6.44 for one magnet. In the US I couldn’t find the free one, the price for a fully customisable magnet is $9.99 .
Not a lot of money in either case, considering this magnet will travel with you wherever you go, showing people your book cover in full colour and where you can buy it.
Keep the message simple, not a lot of text, just a web address ideally or the book shop you mainly want to send people to buy your book.
The magnet will be a talking point when you pick up friends or drop off kids at sports practice.
And if you’re not sure which side of the car the magnet to place on, simply get 2, one for each side. 🙂
PS: I’m not affiliated with Vistaprint, but they do have good offers that I thought I’d share here and I have used them in the past and would recommend them.