If you are not fussy and want to use one of the more expensive housing brands like Jagwire or Shimano, standard cable housing is pretty inexpensive bought online. I'm still working with a 25 foot roll of shifter cable purchased from Nashbar more than a decade ago. This happens to be a source located one town away who sells on eBay.
12 foot roll of 5mm shift cable $16 postpaid https://www.ebay.com/itm/291917358195 same company 25 foot roll $25 postpaid https://www.ebay.com/itm/391259402350