Tronsmart's 2-pack of USB-C to Micro USB adapters are currently on sale over at Amazon. You can pick up two of these adapters for just $2.99 with the promo code CIR2YM5X at checkout. That's a pretty good price for a couple of USB-C adapters, which will keep you from needing to buy new cables with a new smartphone that uses USB-C.
These adapters from Tronsmart are pretty simple. These sport female micro USB ports and male USB-C ports, so you can put these on your existing micro USB cables and turn them into USB-C cables which will work with newer smartphones like the Galaxy S9, LG G7 ThinQ, HTC U12 Plus and many other devices out there. While these aren't the same as having actual USB-C cables, they are definitely much cheaper then buying a new USB-C cable, which will cost you around $7 when they are on sale. Tronsmart does sell these adapters in black or white, so you can get one that matches your existing cables, if you really want. These can also come in handy if you have a number of devices that still use micro USB, so you can just take the adapter off to charge those micro USB devices, instead of needing to carry other cables with you everywhere you go.
This item is also eligible for Amazon Prime's free shipping. Which includes free two-day shipping, and pretty cheap (usually $3.99) next-day shipping if you need it extra fast. If you are not an Amazon Prime member yet, you can sign up for a free trial of Amazon Prime. You'll get 30 days free (if you're a student, you get 6 months), and then it's $119/year (or $49/year for students). Prime members do get a whole lot more than just free shipping though, so be sure to check out the full benefits.