Help me to understand what adapter you need. Did you decide which filter system you want to use? There are several slot in filter systems in use, Cokin uses letters, Haida and others use the size in Millimeters to define series (75, 100, 150). Adapter rings for the filter holders vary between the different filter lines.
For all of them you can get cheaper knock-offs from China.
For screw-in filters you don't need any adapter, just the step-up rings to match the largest diameter of your filters.
In the long run I would suggest you have a look at the slot-in types. Especially if you need to use more than one filter (e.g. ND+CPL, ND and GND, several ND's) the slot type is a better solution, although more bulky.