What exactly do you mean by you can’t find them? That is an ambiguous statement. You asked for the manufacturer. This company lists:
Rainbow Markers Set
With The Incredible New Rainbow Markers Set, You Get A Rainbow In Every Stroke! The Interchangeable Cartrides Let You Draw 3 Colors At A Time. Mix And Match The Ten Cartridges For Over 100,000…
Perhaps your question is not explicit enough as to what you’re looking for, because this seems to answer what you asked. If the problem now is that you don’t want to order as large of a quantity as a manufacturer would require, I would suggest working with an ad specialty company and getting them to source the product from overseas. Often they can get smaller quantities. Or, if this is not exactly what you’re looking for, then they can work to get it made for you. It’s entirely possible that whatever you saw was pulled from production for toxicity issues.