Seeing your other post my guess is you are looking for a new second due to Zak telling you parts are not available....what you have to understand is that is manuf speak for we don't put together kits for it and some of the major parts are not available, it does not mean it can't be serviced as long as major parts (body and the like) are in good shape. The thing is most of the service parts are either generic or still used in other regs. One of my favorite second stages is an old MR-12 that was made while they were still Voit and later AMF (mid to late 60s). Mares will tell you that parts are not available but in reality they are, just not in a complete kit. The main diaphgram is still used in current model reg, the exhaust valve in another and the rest of the parts are generic (orings and the like). The other issue you will have is finding a shop that will do the work, most will give you the same "parts are not availiable" BS either to avoid having to do some legwork or simply to sell you a new reg. This is a perfect reg to start a DIY project, repair it yourself and dive it. Like fishpie said, odds are the only thing it really needs is a LP seat and those are easy to come by and cheap. ($3ish) There is a very stong DIY commmunity here so help is easy to find.
To answer your original question, like the rest have said, you can use pretty much any second stage you like, it will work fine. However, it is likely that it will need to be adjusted to work at it's best with your first stage which means you will need to find a shop to do the work or DIY. Keep that in mind if you are thinking of buying on line. Another issue will be if you use a different brand, you may have trouble getting all of it serviced at one shop. Many shops will not service a brand they do not sell.....another good reason to DIY.
