I googled pentair sand filter wrench and got several results with varying prices and materials. Since they did not all look the same I did not post the link here. Try the google search for the specific model filter you have and see if the results show your old wrench.
You may not need to change the sand in the filter. If it is not filtering like it should it may be clumped or channeled. You can deep clean and re-seat the sand fairly easily once you get the top off the filter. To clean the sand, turn off the valves to and from the sand filter and drain the water from the filter. Then open up the top and gently work a running water hose down into the sand, being careful not to break or disturb the laterals down under the sand. Allow the water to flush out the dirt and debris and repeat, moving the hose to another section of the filter until the whole filter has been flushed. Make sure the filter is full of water before putting the top back on.