rotted sills

In my 1930 built house there are some back rooms that were add ons.. very close t the ground with no crawl space. outside there is a concrete ditch to drive water away from the foundation but the sill has rotted away from the entire back of the house, also carpenter ants and probably some termites although i see no active bugs.  we tore out some flooring and exposed some walls i would really love to have a solid 6x6 pvc plastic post to use as a new sill it would not absorb any water and i could screw all walls and plywood to it... but i cannot find a product that ships to home for this purpose.. any ideas on what  can be done. the water is more than likely going to continue coming in contact with whatever we put back as i cannot afford expensive french drains yet....  but i desperately want to get this section of the house done so i can live in it in a few months.. any suggestions would be appreciated.. i know this is not natural building but i couldn't find a topic that matched my subject... in the forum lists and it would not let me post otherwise..

Maybe the combined minds here can help you find more natural solutions to your issues

The first thing I would say is that, french drains or no, you need to do a lot of digging to get the ground surface behind the house sloping away for at least a few feet, and positively draining around to the downhill side. No matter what materials you use in the house, you will probably have damp until you address that. It may sound like an impossible job, but if you are in decent shape and the ground is not very rocky, you should be able to dig a few feet a day, and before long it will be done.

When you replace the failing retaining wall, don't succumb to "neatness" and build it vertical; build it with a slope back into the hill of something like 1 in 6 (4" for a 2' wall). This will make it much more stable without the expense of deep footings and rebar.

You will likely need to use some sort of waterproof material at the base of your walls, be it plastic or masonry. Even rot-resistant wood will wick moisture up into the rest of the wall.
Alternatives that occur to me are making the bottom 6-8" concrete or masonry, finished smooth on the inside, with your wooden walls on top, and accept that there is a strip at the base that is a different material.
If you can dig the ground outside to 6" below floor level sloping away, then you can build masonry foundation up to floor level and conventional walls from the floor up.
If you can deal with a step up into the back room(s), you could raise the floor level in those rooms so that you can have foundation up to floor level with standard walls, and not have to dig so deep outside.

It looks like you will have to completely tear up and replace the floor structure, so that gives you the chance to do more digging (!) and get a crawl space where you can put vapor barrier down on the dirt. As I found as a teenager helping my father upgrade under the bedroom wing of the house he had built, that vapor barrier makes a huge difference by itself.
Another possibility would be to put in gravel, vapor barrier and a concrete slab.

I look forward to other ideas on this case.

thank you for the advice... i think i will dig out way back into the bank and do as you have suggested...  as for making the room a step up unfortunately its already a step up in those back rooms from the front...  i can however dig out some and put down some vapor barrier. i worked yesterday with a friend and got 6 inch gutters and down spouts put on those two sides to funnel some of the water away.. it looked very  nice, will take some photos today to show the progress...   very sore going over today to try and tear up some more of the floor i hate that the floors were not in better shape back in that room would have loved to have salvaged more of the boards. i kept a few to fill in some huge vents that used to service the oil stove but most of my boards were wet damaged and eaten underneath a bit..  im going to post something about my chimney next, i had a chimney guy come out and unfourtunatly my  chimney top that sticks thru the roof needs some work before they could even put in a liner..  thinking about trying to build out a wood surround and then have the topper and liner put in...  lots and lots of work.. thanks so much for any advice you can give me i am an avid do it yourselfer.. if the contractor this weekend comes back too high i will be attempting the sill replacement myself as well..  i cant stand to pay to have something done it goes against my better judgement..  including a few photos of the gutters it was way past dark when we got done.  ...   talk later... thanks again..