Every one is different.. And financial situations chose whats good for them..
You can buy:
polar boards- thin boards with clips
core boards- there's a member here who sells them - again a thin core board
comic boards- can order them online - these are thinner and some use 2-3 at a time for stiffness
Picture mat boards- you can get leftovers and cut to size from the framing stores
Ruler fold- free - and you just wrap your fabric around one of your rulers and fold
Bolt boards- free- joannes, hobby lobby and LQS will give them away as they will be trashed
I ruler fold anything less than 2.5 yards
I keep anything more than 3 yards on the fabric bolts