Don't buy expensive "shocks" which are nothing but glorified springs, just find springs of a suitable strength. Leaf springs would be fine as well.
Cheap springs can be found on a junk yard - car seats, beds etc all have either coiled springs or at least steel frames that makes good leaf springs.
I have made several craft knives out of the flat-steel frame from a bed - very good steel quality.
Why are your 'bot so heavy anyway?