Clay cat litter is cheaper and the same sort of stuff!
Obviously check for any buried services as far as you can. There are sites like this for main stream power & gas, but not everyone is on there:
LinesearchbeforeUdig (LSBUD) is a free internet based search enquiry system available 24/7 to make asset enquiries online with over 3 million enquiries per year
lsbud.co.uk
Also remember that the main factor is soil conductivity is moisture, so if you can go deeper (say 8' = 2.4m rod, typically in two sections) you will generally do better than two short ones in parallel as more chance of the soil remaining moist.
I have used M16 stainless studding as an earth rod driven in like a giant drill (SDS adaptor and a 1/2" drive stud runner), but obviously you get purpose made ones and matching bit like access boxes, etc. For example:
kingsmillindustries.com
Browse our range of Earthing & Lightning protection equipment and products online at AN Wallis including earth Rods, Bars, U Bolt Clamps and more.
www.an-wallis.co.uk