When our brewery is operational (mid September) you'll be able to exchange your ball lock corny keg for one filled with our beer. You'll just need to give us notice and we'll do the rest. We'll be in Astoria. singlecutbeer.com
If you just want to test kegs,pressure,hoses,faucet and/or practice add water and make seltzer.
But carb the H2O like you would beer,so you can get familiar.
Don't go and force carb it at a high pressure for a week then tap it.You'll have a gusher.