Why would the source code be lost? This is news to me.

why wouldn't it be? losing it is pretty much the standard procedure - only very, very few old(er) games have their source code available (be it public or non-public). for whatever reason, "release your source code to the public once it has no more commercial value so your game(s) would live forever" is not a very popular way of ending a games' lifecycle.

lets see whether I can dig up some examples of old games that actually do have their source available;
-[url=https://en.wikipedia.org/wiki/Doom_(1993_video_game)]Doom[/url] (public. id soft were one of the few who understood this from day one)
-[url=https://en.wikipedia.org/wiki/Marathon_(video_game)]Marathon[/url] (public)
-Duke3D (public)
-Shadow Warrior (public)
-[url=https://en.wikipedia.org/wiki/Blood_(video_game)]Blood[/url] (non-public, owner decided to not release)
-Rise of the Triad (public)
-all Dark games (the code exists, but current status unknown, and is still owned by two separate entities)
-[url=https://en.wikipedia.org/wiki/Thief:_Deadly_Shadows]Thief Deadly Shadows[/url] (the fairly recent xbox360 release suggests that the code still exists)
-Turok (NightDive apparently having the code, making the enhanced GOG release possible)
-most Unreal1 powered games

those are ones that I remember, you can continue here - but you can see right away that the list has maybe 200 entries? considering the thousands, if not tenths of thousands games released in the last quarter of the century, it's pretty much nothing. that's why fans of classic games go through the painstaking process of reverse engineering or recreating them from scratch - unless they are C&D'd by the very same idiots who have managed to lose the source code in the first place (many companies are completely nuts about this - they don't care about their games or the source code, but will relentlessly attack anyone who they feel is even loking at their IP in a wrong way).

you are perhaps confusing the SS1 source code with SS2 source code, which was known to be in EA's possession, and was also leaked a couple of years ago.

Problem is OS9 is so far removed from OSX its not even remotely the same operating system, it would be like comparing windows to unix. To emulate it, you need a copy of OS9 and a bios rom from a PowerPC mac, and even then, sheepshaver is very finicky on what it chooses to run well. A lot doesn't.

But being a dos game, your best bet is to run it on DosBox, unless you mean this enhanced edition, in which case your real best bet, is wine, or more specifically wineskin which simplifies the process for OSX users immensely and is used by GOG for SS2.

As voodoo said, source code surfacing is extremely, extremely rare. A few more you can add to the list, that I know, Star Control 2 which made possible The Urquan Masters Project and the spin off project, The Urquan Masters HD. Also wing commander 1 is being held safely on an offline archive by the fine folks over at wcnews.com. Unfortunately it can't be publicly available, but it is at least safe.

You get to be a PC gamer, or a Mac user, not both. You chose... poorly.

And, why exactly do you say this? If you're gonna troll, you best try a bit harder than that. The game was originally released for Windows and Mac, so my request is not outlandish. Additionally, there are plenty of games for Mac users and there is no reason not to request more. I consider myself a very casual gamer who strongly prefers Mac and has come to absolutely HATE Windows OS (i.e. "Program is Not Responding" OS). You choose Windows? That's your fail, not mine. Mac makes me happy. Windows makes me miserable. I chose wisely.

P.S. I played and finished (and loved) SS1 a long time ago. Would just like to give the enhanced edition a whirl in my OS of choice without BS and glitchy wrappers.

The game was released for dos and Mac OS classic which is of no relation to OSX. Windows 3.1 was lousy then. :p Mac OS classic versions of games tended to be better then their dos counterparts graphically as a game had to had SVGA graphics, where as the dos versions often were VGA only. In System Shock's case the Mac version ran consistently at a higher frame rate then Dos and the soundtrack was fully pre-recorded, and a far cry above soundblaster bleeps and bloops. Ironically its the Dos version that is easy to run on OSX thanks to dosbox.