For fish, you can get the length-weight (L-W) relationship for most fish species in www.fishbase.org . When there is not available info for a specific species, may be can work try with a close-related species. LW relationship included in this source come from peer-reviewed papers in most cases. Fishbase gives the max weight reported for the species, but the LW relationship is also very convenient because allow you to know the weight across its entire life.
In an African context it is considered one of the most cited resources. I also recommend reference books by Estes (however the above really is a remarkable work, originally based upon Smithers and Skinner).