Robinsons do two types. One is like a upside down bucket for 20 (ish) pounds and then they also do a step one for just under 30. Both are very good IMO.
Make one. My 10yo son made one for me no problem. Or use a plastic chair, but not in the mud as I did, it was very embarrasing when the legs sunk into the ground.
The problem is since breaking my back I cannot get on from the ground due to lack of strength/flexibility. I don't hack now. I usually use one of the jump blocks in the school but am finding it too low. Asked OH to make me one - He wasn't having any of it!