Old useless pallets maybe?
barrels, logs, straw bales.
Hay bails, or even use old boxes
Omg thanks everyone gonna try them out and I'll let ye no thanks a million x
You can stack hay cubes as well
If you can get hold of old barrels/drums you can drill holes in them and make/buy cups to make adjustable jumps
Go to YouTube and search Budget Equestrian she has really good videos on how to make jumps for under £30!
If you can find logs/long sticks/tyres they could be useful:)
Use crates and stack the poles on top of them x
Old tree stumps , little wooden blocks (that you can stack up ) the sides of fences and small barrels that you find around the yard , and also buckets that are strong enough