You can only order the flex cable from EBAY if they send, canon Malaysia don't sell individual parts, you would have to send it to canon to repair and no photography shop carry it.

My flex cable for the aperture diaphragm on the 17-85 is spoiled and they told me they don't sell just the flex cable as they will fix the whole aperture diaphragm with the flex cable. Although it is not the same lens , i am giving my input just for your reference, it cost 330 for parts and 120 workmanship. This is a very common problem with this lens as you will see.

Also i googled canon flex cable on ebay, apparently i saw flex cables for 17-40, 1785 and many more, so it is a common problem for these flex cables to break due to wear and tear. 1785 seems to be the most commonly failed lens. EBAY sellers also sell readily soldered flex cable to parts which you can just change if you are not good at soldering and this is also how canon does it after i query them since 330 is absurd for a flex cable.

So conclusion if you want cheap and good with soldering, order flex cable from ebay, i saw sellers from hk and china. If not good get it ready with parts soldered on. Second option is piggy back people looking to buy new body, offer to buy the 1855 lens from them and the new one has stm which will be useful for video if you upgrade the camera body later on. (IMHO 200-300 would be a fair price for new).

You can further your queries at the Canon DSLR thread here see if anyone else have anymore inputs. https://forum.lowyat.net/topic/3293955

Thats all, all the best.