you can get cheap waterproof hay bale covers, i bought scary ones to get mine used to fillers and they were only about £5.99..for poles i just use wood, and wings i use plastic outdoor chairs :)

Pop carts, just stack them up on each other, logs, tires, i also use cones, barrels.

Most answers on this subject seem to be covered but for a water jump, buy a blue or black plastic dust sheet cover from DIY store. U can create it as good or bad as you want by digging a small trench/type hole so you can fill it in with water. U could put a pole over it or not.

Another tip use old plastic feed bags to tie onto jumps to make really scary or an old rug or your coat to lay on pole. If horse well advanced in jumping (not younster) then if it jumps this at home it will jump practically anything at a competition!

Garlic tubs under poles or traffic cones are excellent as an alternative to fillers. 

Skirting boards are excellent alternatives to planks just a liquer of paint and away you go together with using large brackets that you would put shelfs up with so they stay on jump cups