There are a ton of toys that have some catnip built-in. However, very few of them are refillable. Some that are:
An alternative is catnip balls, which roll around on their own, and don't really need a plastic container. (though note that they don't get great reviews)
Another alternative is to get some sort of "spice/herb shaker", and have the human sprinkle some out as needed. Just get ones like this, which very large holes, instead of ones like this with much smaller holes. Otherwise you'll have trouble getting a sufficient amount of catnip out.