Several things:
1. The easiest and fastest (but most expensive) route would be to contact BGE Corporate in Tucker, GA and order a replacement. That'll get you going quickly, but.....
2. Given how the demise of the lid occurred, there's a good chance that you'll need additional hardware (such as the lid hinge. Check for out of round, rust, etc., and be prepared to buy - they are not cheap. Sometimes they show up on eBay.
3. Given the age of the egg - you mentioned it's an older unit - check the condition of the fire ring and firebox. If you have to purchase either of these, plus the lid, then your price approaches that of a new Egg, which comes with a limited lifetime warranty on the ceramics anyway.
4. Scour Craigslist and search for "medium Egg lid". They show from time to time, but not nearly as often as the large, since the large is the most popular size.
My advice?? Personally I'd pass on it. It likely has the old-style hinge mechanism. If it's a really old unit, then its best years are far behind it, and will be much more of a money pit than you're prepared for. Wait for a complete unit on Craigslist ....... they generally go for $400-$500 for a complete good used Medium here in metro Atlanta. I only paid $400 for my large last summer ........... which MAY have been used 5 times.