Hello lhhoback,
I've looked into this and there are a couple factors to consider (hopefully others will chime in).
The YS holder is a plastic component that holds the lamp and allows it to be connected to its YS connector counterpart. In the attached picture (source: Adorama website), the holder is the part that grabs onto the lamp (not the rest: screw knob and whatever is in the lower half) . By itself the holder is useless, unless you have additional parts for this holder to connect to the camera or flash.
The most common thing to buy would be a flexible arm that would connect to the camera base. This can get complicated and expensive since the Sealife's flash base (which attaches the camera to the flash arm) does not have any connectors to attach anything else. In order to do this you either have to go DIY or get into pro equipment (get a base that will provide more flexibility, such as stuff from Beneath the Sea and others mentioned on SB forums, then add a flexible arm with the connector that will attached to the YS hodler).
Unfortunately, IMHO a focus light that is attached independently from the flash is a pain in the neck: ideally you want the focus light beam to aim at the center of where the flash beam will hit. If you have an independent flexible arm, that yet another thing you need to adjust underwater. Since focus light will be most useful in low light conditions (ex.: caves or night dives), the photographer/diver will already have to deal with adjusting the flash position. By the time all is ready, the critter will have vanished!
Better option would be to buy a holder that would enable you to attach the focus light on the flash so that it is pre-aligned. I haven't found one for the Sealife flash, if anyone has, please let us know. I ended up making one myself.
If your LDS has someone knowledgeable, they might be able to help you out.
