Fanless PCs used to be so expensive. Not anymore. Thanks to a multitude of 35W processors and cheaper CPU coolers, building a totally passive computer has never been so easy. If you don't care for PC assembling, take a look at Quiet PC's Nofan A810a Silent PC and Nofan A830a Silent PC featuring AMD's latest Athlon (35W Ryzen 3 2200GE and Ryzen 5 2400GE are also available).
Quiet PC is a highly ethical company and probably the UK's best PC builder (check the amazing reviews). Prices start at £627, shipping to 90 countries worldwide.
Source: Quiet PC