Contact an archery range or even an archery store. Get a discard aluminum (or carbon fiber) arrow, complete with intact nock tip and front end ferrule where the point screws in. If the shaft is bent or broken you may have to heat the nock &/or ferrule in boiling water to release the glue and cut the shaft to the correct ltngth. Use the same kind of glue to reattatch the ferrule on the shortened shaft.
Try to get the largest diameter shaft you can for greater strength.
Shorten the shaft to 18" with a small pipe cutter or have an archery shop cut it for you with their nifty power cutters. When using the arrow shaft tent pole just run the guy line through the nock's string groove.
Before good, light tent stakes came along I used aluminum arrow shafts and ferrules for tent stakes. I removed the plastic nocks to expose the pointed shaft back end, cut the other end of the shaft to length and re-glued the ferrule. Then I screwed on an aluminum screen door tab into the ferrul as the stake's top end to retain the guy cord. LUCKILY the thread on the aluminum screws of the screen door tabs was the same as the ferrules' thread! I couldn't believe my luck.