Holography requires that two beams interfere at the plate. This is usually accomplished with a beam splitter.
A reflective beam splitter is a partially silvered mirror. It can be made so as to reflect a percentage of the light and transmit the rest. Common beam splitters come in 90/10, 70/30 and 50/50 splits.
There are also variable beamsplitters. They consist of a partially silvered mirror that changes density from one place to another. The two types commonly available are linear and circular. The require a stable mount that will allow the ratio to be adjusted without changing the beam direction.
Most commercially available beamsplitters are variable neutral density splitters. As such they are made on thin glass. It is common for hobbyists to laminate a piece of glass to the beam splitter to make it thicker. This separates the reflection from the back surface from the reflected beam in order to allow it to be blocked. Any index matching glue can be used. Norland makes a range of UV cured index matching adhesives that are suitable.
When placing a reflective beamsplitter make sure to place the reflective side towards the laser.
Cube beam splitters are used when higher damage thresholds are needed. They are more expensive but there is no problem with the ghost beam. Cube beamsplitters are available in Polarizing and Non-Polarizing varieties.
A polarizing cube beamsplitter can be made variable by having 2 1/2 wave plates. One on the input beam and the other on the straight through beam. The input polarization is rotated to get the desired beam ratio and the output polarization is rotated to match the reflected polarization.
Cube Beamsplitters are often AR coated.
Pellicle Beam Splitters are not suitable for holography owing to the fact that their thickness (2um) makes it impossible to block the ghost beam from the back surface.