places like B&Q & Wickes etc will have tannalised timber and they will be able to cut it to size for you. any builders merchants will also be able to help supply the wood.
welcome to GoY too :o)

Not sure if you could rest or fix wood supports to your path,as the weight of the soil would surely push them over..you could use log roll,which would fit on the inner side of your path,and is used for that purpose,and easy to install as it's sold in lengths,which you just trim to size,if need be.. It doesn't last forever,but ours has only been replaced once in 15 years,before rotting,and it's reasonably priced..if you decide to try it,look for the best quality available..I'm not sure if varying heights are available,or if it's only a standard height...good luck,and welcome to GoY too :o)