Sure, there wasn't an internet to speak of in 1985 but there were still plenty of places like today's Newegg to order your computer stuff from. You just had to pick up a phone and call.
The above ad is from the December 1984/January 1985 issue of Commodore Power/Play and is for one of many such places in the 1980s called Computer Warehouse. A Commodore 64 and disk drive would cost you over $400 then.
Read more: https://www.megalextoria.com/wordpress/index.php/2018/07/27/computer-warehouse-1985/