To expand a bit on what Brad said.
The Magazine Lee Metford (& Lee Enfield) used a leather sling. About 49 inches long. The first couple of shots on this posting shows that type of sling.
http://www.milsurpafterhours.com/bb/vie ... f=3&t=6197
These (in shortened form - about 44 inches) carried over to the SMLE. At least most of the pictures I've seen of the very early SMLE MkI had the leather sling. I'm going to disagree with Brad on the date for the web slinging. I believe it was around 1903. Initially it was a trial version with the current sling being introduced (and remaining unchanged) in 1908 with the Pattern 1908 web equipment. The first couple of photos in this post show the Pattern 1908 sling.
http://www.milsurpafterhours.com/bb/vie ... f=3&t=6522
The MLM/MLE slings are rare but are around. If authenticity is not a major issue any competent leatherworker should be able to make you one. Not sure about reproductions but wouldn't surprise me if they were available.
Pre 1908 web slings ? Good luck. If you've got deep pockets, lots of patience and the luck of the irish you
may find one.
Pattern 1908 web slings are very common. If you've got an early MkIII you might be lucky enough to get an early dated one but expect to pay for it.
Reproductions do exist, most marked with dates around 1914, so be a bit wary of dated slings in excellent condition.
Be aware they come in many colours. Rifle regiments in the British army used to blacken theirs and the RAF ones are blue. Some were whitened for parade use.
Where to find them ? Depends where you are. Ebay usually has a few of the web slings and occasionally some of the leather.
Almost forgot the leather slings were re-introduced in 1914 when supply of web equipment couldn't keep up with demand. Known as Pattern 14 they were also produced in WW2.