Do you have access to woods with a lot of pine needles, or fallen leaves, especially oak leaves? You can mix a lot of that into the soil.
Here, needles from long-leaved pine trees are gathered and sold as pine straw (a weed- suppressing mulch). If pine straw is available to you, it's a great acidic additive to your soil. To keep the soil acidic, use pine needles as a mulch too.
Bark-based soil additives, or bark-based "top soil", will also make your soil acidic.
Other than that, the previous recommendations are good.