For Cories and Shrimp? Don't go the dirt route unless you absolutely feel the need to save tons of cash. Otherwise, your opportunities are limitless.
You could do anything from Safe T Sorb to pool filter sand to black blasting grit. That would cost probably $5-$10 to do the entire tank. Could use root tabs for your plants that need them.
Fluval Plant/Shrimp Stratum would lower your pH a bit for about a year but would cost $30-$50 for your entire tank. Would still need root tabs.
UP Aqua Shrimp Sand would lower your pH end up costing you easily $100 or more. Root tabs still required. It would be cheaper to get ADA Aquasoil if you went that route.