Concrete blocks, old tires, tree stumps, upside down buckets....all you can place a "rail" on...
Caution though what ever you use allows the "rail" to freely and easily roll away so if you hit it, it not trap the animal and reduces risk of injury. Please, please do not use PVC poles
..they shatter to easily with contact/impact of hoof.
Over time, PVC ages to become brittle whether indoor or outdoor arena used. They are not
"cheap" to purchase and old ones laying around...just don't.
It is why those who use PVC fence for horses must have a specially made and treated fence so as not to impale your horse on a rail broken from just a lean-on encounter! Ouch!
Instead use landscaping timber or split rail fence rails if you don't want to invest in actual poles.
10' minimum long would be my choice of length as they give you enough space to not catch a stirrup on a standard if you are not centered going over the jump.
Depending upon where you live...lodgepole pines make for great rails, long & straight.
Look at your natural environment...brush from trimmed trees makes for a natural jump as long as it is stacked and steady when placed, not wobble around.
Hay/straw bales make for fantastic jump standards, "rails", and to make a chute to guide the horse in and help the over.
Barrels make for good jumps, plastic ones on their sides or as a jump standard...
Again, focus on the ability or the rail to move freely if tapped/hit by the horse for whatever the reason.
Never fasten or stop the easy go-away of the rail as that causes terrible injuries to horse or human in contact.