Find your closest lumber yard --try local, not box stores. Always good idea to have a sample pole to show them --I used to buy 4x4 and have the corners pealed off --cost was about $15 --$10 for the pole and $5 for labor --I used 12 foot poles too. Best time to do this is in January. Lumber yards are not getting a lot of business in January when building is slow – if the lumber yard is unable to accommodate your request --ask where you can have it done --sometimes they know a builder who will custom cut wood —again, January is a slow time for builders. Another possibility (and I have done this) is to contact the local high school if it still has a wood shop class. If there is one, find out the name of the teacher and when he/she is free during the school day. Call and ask if you can come in to discuss a small custom cutting wood project. Be prepared to offer a donation to the school or pay the teacher out-right if he/she is going to do the project after school hours. [I worked at the school, so it wasn’t too hard to get my poles shaved]. Last choice is to do it yourself. You can, if you are handy with a Skil saw, “free hand” taking corners off a square pole --but it’s difficult to get a good result (I’m an old lady and holding the saw was not easy for me, but I got the job done) --a table saw is ideal --and the poles need to be cut true, no bend.
If poles/jumps are something you plan to build a lot of —in 20+years I eventually built a full 15 obstacle course of wing standards, elements, and poles --buy a table saw, chop saw, and drill press (although the last two I think you can rent --maybe you can rent a table saw too, but they take some skill to use, I think.
I would expect to pay $20 a pole —having said that, I am still using 30 year old poles in my ring . . .buy as many as you can afford if you find someone who will do the work for you.