I was tempted to suggest beetroot, but wikifying trimethyl glycine yielded this:
"TMG is an organic osmolyte that occurs in high concentrations (10s of millimolar) in many marine invertebrates, such as crustaceans and molluscs. It serves as a potent appetitive attractant to generalist carnivores such as the predatory sea-slug Pleurobranchaea californica.[5]"
So just whizzing up whatever shellfish the market-stall chap (or the supermarket) has to sell off cheap near the end of the day may well be the budget way forward.
Which saves me from embarrassing myself with a bright red, stain-all, catch-naught "Additive". Which is nice.
Afterthought: Chinese Oyster sauce???