Most of the downstream, dual adjustment regs use a fairly standard rubber seat in the second stage that can be flipped.
With the balanced single adjustment second stage designs, the seats are normally either in the poppet assembly or bonded to the poppet assembly and are not generic or easily replaced wthout access to the proper parts.
But...if you are real careful and are good with your hands you can take your wife's emery board and resurface them. You need to be very careful to keep it absolutley square and flat and to stop when you have sanded just enough to remove the set ring. This also usually only works once as the rubber on the poppet is too thin to tolerate refacing twice. It is definitely not manufacturer approved and is an "at or your own risk" kind of thing. It will also make your wife mad as it leaves a black smear on her emery board. The thinner refaced poppet seat may also cause some problems in single adjustment regs as it can affect lever height and consequently cause problems with the flow volume available from the second stage. I have done this with vintage regs where the parts are no longer available but highly recommend getting a new seat or poppet assembly if at all possible.
As for o-rings, in most simple standard unbalanced piston designs the dynamic head and piston stem o-rings are not exposed to full tank pressure so the normal garden variety o-rings in the plumbing department are usually (but not guarenteed to be) hard enough to work with the 120-145 psi pressures involved assuming they are correctly sized.
Balanced regulators are a different story as the stem o-ring will be subjected to full tank pressures of up to 3300 psi. So it needs to be adequately hard and precisely sized to prevent an o-ring pinch bewteen the body and the piston stem and needs to be very durable as well to tolerate the 1500 or so cycles per dive that it sees. There is not much room for error for an o-ring in that application and getting a generic one from a third party source can be a problem if you do not know the design specifications for the required o-ring.
The static o-rings in the first and second stages normally just need to be cleaned, inspected and re-lubricated and do not need to be replaced every year, so parts availablity is not as big an issue there.
With either style of first stage you also need to understand some basics regarding IP pressure tests to ensure things are working properly. You also need to understand the requirements of adjusting both single and dual adjustment second stages to do it properly and safely.
Contrary to what has been suggested, HP seats can and do fail over time and usually result in a slow creep in Intermediate Pressure that ultimately causes a freeflow in the second stage. The knife edge on the piston is not in contact with the seat when the reg is not pressurized, but the seat does see an awful lot of cycles per dive (1500 or more). Some of the flat variety of HP seats in piston second stages can be flipped. Concave or conical seats cannot and need to be replaced.
The cautionary statement here though is that the consequences of the failure of an improperly or inadequately serviced reg at the wrong time can be severe and fatal, so it really is important to know what you are doing and know what to look for when inspecting a reg.
In my opinion reading about servicing a reg and then doing it is just like reading about wreck penetration and then doing it - you may know the theory but the devil is in the details and you are really asking for trouble if you do it without formal instruction.
If you just have to do your own servicing, get some training from a tech on your specific reg if possible or at least get some good reading on the subject first. The following is a very good guide that will help you understand the mechanical principles involved although the author clearly states is not designed or intended for do-it-yourselfers. Just like the guys who write books on wreck diving - they also state you need training to go with the reading.
http://www.scubatools.com/RegSavvy.html
