Peat would probably be the easiest additive to add the acid as it breaks down. Past that, I'd probably head out to any big box store or garden center and pick up a bit of Hollytone (fertilizer), MirAcid (fertilixer), Aluminum Sulphate or Dusting Sulphur (both additives). If you have a pH meter or have enough seeds to have a test batch, you can also dilute some vinegar and use that as a portion of your watering regimen.

I second makalu's suggestion of peat. I mixed some in with potting soil to increase acidity for gardenia, an acid-loving plant. It did the trick.

First I'd contact the county agent and get some pH testing liquid. Around $4 a bottle. Follow the instructions to lower the pH. Soil in pots are such small quanities that it's easy to overdo it. (3lbs sulphur lowers pH by one unit per 1000 sq.ft.). I'd stick with slightly acid peat moss, or vinegar at the rate of i Tablespoon/ gallon for just one watering.
The Great Lakes and adjoining land are all pH 7.4, just interested, what's yours?
Doug