I bought a whole set off ebay that were very economical. They lasted about 6 years before looking ropey and 10 before needing replacement. Obviously, better ones would have lasted longer, but they were cheap and cheerful.
I bought shorter ones LOL, as my arena is quite small anyway and if I'm jumping 'properly' I hire a big arena with BS jumps, so they were just for schooling exercises.
Timber yards or places like Mole Valley Farmers sell suitable wooden poles and they are cheaper than anything described by the seller as a 'jump pole'. You need to paint them if you want nice colours, but they are perfect for the job.
ETA Also all sorts of bits of wood come up for sale on Gumtree, just look for wooden poles rather than just jump poles. I've picked up 10 free brand new fence posts on there so there is some decent stuff (also some stuff that people should just take to the tip!).
I got 9 used, really good, painted wooden ones for £10 (in total ) on ebay. That was a one off though and they were better than they looked in the photo, but worth just keeping an eye on there.