I did a quick check, and Freestyle was the only place I found that still offered filters for below the lens use. You need a filter holder, like the one for multigrade filters to use these effectively. It would be a real pain to try to hold them by hend. Most enlargers come with a holder that you can swing under the lens, but you can get one that attaches to the lens as part of the multigrade filter kit. You have to be careful in handling these filters. They are pretty fragile, and cleaning is an uncertain thing. You also can use at most three filters at a time. (The kits come with enough values so that this is not much of a problem.)