Originally Posted by Scotia

There are a number of ways you can go about it e.g.

1) Contact a demolition contractor and see if they have any old floor joist available.

Normally they will keep the best for re-cycling back into buildings but sometimes they come across stuff that won't be able to be used for such a thing, but would very suitable to your purpose.

This type of timber is generally well seasoned as opposed to much of which you buy today, so it should last for quite a few years.

2) You could use tanalised timber which as far as I am aware does not leech the preservative because it was placed by a vacuum process as opposed to painting or dipping.

A tip here would be to buy from a fencing supplier rather than a construction timber merchant, they use a lower quality wood, i.e. it will have some 'shakes in it and is probably be heavily knotted, which is OK for your purpose but not for construction purposes.

3) Then if you want to use materials other than timber you could use concrete blocks or path edgings.

The choice is your, but I would guess each of these suggestions are cheaper than cedar.