Stush,
You should find a local NAPA Autoparts store and check out their NAPA oil dry. The bag should say 100% Fuller's Earth and also be labeled as usable for cat litter, soil amendment, etc. That would make a great addition to your bed (and should be available in PA). I would also add course sand that has been washed, typically a builder's sand is available and not too expensive. Just make sure that nothing has been added to the sand. Avoid white sand or very fine sand. I also use a fine quartz gravel, around 1/4" in size if you can find it.
Here's a quick tip for you. Anything that you want to add to your semp bed, such as I've mentioned, should be tested. Add a sample to a jar of water and let it sit for a week or so after a quick rinsing. The water should remain clear and the medium you are testing should not get mushy.
Lastly, pine bark fines are a great addition. They will decompose slowly but do not retain a whole lot of water. Around here, I shop the suppliers to find "mini nuggets", then check to find the vendor with the smallest nuggets. Then I sift them thru hardware cloth (stainless steel mesh). I like the 1/4" mesh for this purpose. The bigger stuff left over goes for mulch elsewhere. Pine bark will stain the water brown if you test it, but is otherwise OK after a quick rinse.
You don't need a lot of soil in the bed. You can grow the semps in mostly gravel, sand, FE and pine bark. The use of materials that don't decompose or break down much means that your semps will not have to be replanted as often due to frost heave nor will you have to add mre media to the bed more often.
