Rotton window frames in old lean to/conservatory

[Twist1972]

Hi all, I have a lean to/conservatory attached to my kitchen and some of the windows frames have become rotten, and some disappeared completely. I'm not sure what I will do with this lean-to in the long term, so don't want to spend a lot of money repairing it.
A couple of the window panes are in danger of falling out.
I need to replace the wood to keep the windows in and to look presentable for the time being. I've attached a photo of the worst window, any ideas where I can buy the wood strips, and the technique/ tools I need to repair please?

 

[blup]

Get treated timber strips/battens from any of the big sheds and rip down to size. Tack in either side of the pane to keep it in place as a temporary measure