Check with Spectra Stable Isotopes 
http://www.spectrastableisotopes.com/Home.asp.  They grow algae with 13CO2 
and may be able to sell you unfractionated 13C biomass.

Another alternative is to consider whether you can pull off the study by 
designing it around C3/C4 materials.  For instance, if your eels start out 
with an isotope ratio near C3, then feed your earthworms C4 (corn) and then 
watch the eels shift the 20 permil as they consume the worms.  Of course, 
it works in reverse (C3 earthworms for C4-like eels).  This is probably the 
cheapest way to do it if your experiment can tolerate it.

Tom Brenna

At 10:04 AM 6/12/2008, you wrote:
>Hi all
>
>I am evaluating turnover rates of fish (American eel) tissues. I am going 
>to culture about 7
>pounds of earthworms to feed the eels and I would like to enrich the 
>earthworms in both 15N and
>13C. I plan to add 99.9% 15N ammonium chloride to the worm feed, but am 
>having difficulty
>identifying a cost effective carbon label (13C glucose will be too 
>expensive in the quantity I
>anticipate needing). Cost is a big limiting factor for me; can anyone 
>point me to a low cost carbon
>label? Or do suppliers of isotopically labeled organic material exist? Any 
>insight will be greatly
>appreciated.
>
>Many thanks,
>Alyson
>
>
>Alyson Eberhardt
>Jackson Estuarine Laboratory
>University of New Hampshire
>85 Adams Point Road
>Durham, NH 03824