How about these $3 slings?
Ah, Lee, we meet again!
Would these work well on a Mossberg 500?: from
http://www.fredsm14stocks.com/catalog/acc.asp
US GI Web Sling, used - As above, but slightly more used, so good condition only. Green web cotton sling with all metal, generally in fairly clean condition but, as indicated, showing more wear. $3.00
US GI Web Slings, used, As-New - You do not find original GI green web slings in this condition often, as they were used to the hilt, if only to carry the rifle. These are off rifles that seem to be unissued after the new sling was put on, so the slings appear to be close to new, fresh green color, minimal wear. Get one today! $8.00
US GI Web Slings, used, Fair - Good - Pretty-well used US GI green web slings. Show lots of use, but no significant fraying. Comes with Freds Hundred-Mile Guarantee: We promise it will carry your rifle 100 miles, or more. Price is right, huh? Limited quantity. SORRY - OUT OF STOCK, PLEASE DO NOT ORDER... $0.00
US GI Web Slings, used, VG-Exc - Original US GI green web slings, handy for field or competition. Fred never is without one on the range. If fact, right there, inked on his sling, is the name of his rifle, WIDOWMAKER, so he never has to guess which rifle is his... $5.00
WW2 Garand Web Sling - Used - Genuine, original US GI WW2 web sling, with the flat keeper and small buckle. In serviceable grade, these veterans were taken off M14 rifles from the middle 60s, when, believe it or not, they were still being issued. These will be fairly well-used, but will not be excessively frayed. Acutally, in pretty decent shape, about like a car with 80,000 miles - still a lot of life left. Guess I would say, in average condition,for slings 60 years old.
$8.00