Probably not what you want to hear, but you may be out of luck. A quick search for parts for Resmed S7 cpap showed 3200 hits. However, the user manual states that there are no user serviceable parts inside. Repairs and internal servicing should only be performed by an authorized service agent.
So, I figure it is unlikely that you will find the parts anywhere on the internet and if you did there would be no instructions. Further, you would need to have professional help in checking that the pressure was indeed correct. So my recommendation would be to find a Resmed service agent in your country and have them set it up for your prescription. Probably cheaper than a new machine, I would think, but you would need to ask them for an estimate to see if it was worth it. The technology has come a long way since the s7.
Diagnosed September 2009, Respironics System One w/ C-flex and humidifier
Aloha nasal pillows
Not a medical professional, just a patient with severe OSA (31) who has done a lot of reading.