Teaberry, for so many reasons, using the soil mix is one of the most important ingredients of a good SFG. Even if you add ammendments to the soil (you can over ammend soil and have it back fire on you), you will not be able to create the same environment. If you are not going to use raised beds and the mix, you may want to reconsider row planting for those items that are not in raised beds. Doing SFG without raised beds and the soil mix has provided less than desireable results for so many people - me being one of them. Even after reading many times not to do it, I thought I could beat the system and be the one to prove it wrong. I did part of my garden that way, and got the same bad result that others before me had.
Another thought.....watch craigslist or post a wanted ad on there for the wood. There are many people that have a couple extra after a project and are willing to sell them dirt cheap or just give them to you. Another option would be to use something else until the time was right to get the rest of the wood. Bricks, pavers, broken concrete pieces - they all can make good edging.