Isaiah 6:8 (NKJV)
Have you ever had the experience of going to a store to buy something and then finding out that the thing you wanted was not there? I have had this experience more than once. There were times I had planned to get something, and I had even done some research about the thing I wanted to buy, only to then find out that the item was not in the store. What I wanted was somewhere, but it was not where I needed it to be for me to actually buy it. I have even had a store employee tell me that the thing I wanted was available, but it was not where I was, so even though it existed somewhere else it was not available for me.
In both Isaiah 6 and Acts 9 we have stories about people who heard the voice of God. In both cases God was looking for someone to do something. In Isaiah 6, the prophet Isaiah heard God speaking and asking who would go and then speak to the people. Isaiah replied that he was available and that God could send him. In Acts 9 God spoke to a disciple named Ananias to go and pray for Saul of Tarsus, who had just met Jesus on the road to Damascus. The first response from Ananias was to try and explain to God how this Saul was a bad person and dangerous. The good news is that after his initial hesitation Ananias did finally agree to go and pray for Saul.
Being available means more than just existing somewhere, it means being ready for use. God spoke to both Isaiah and Ananias, but at first Ananias was not really willing and available to do what God had said. We sometimes have the same problem in that God will speak to us about something and we think we have to explain the situation to Him, as if He does not already know about it. But, if God asks us to do something we can be sure He will help us to do it. We need to be available, ready, and willing to do whatever it is God asks us to do. Today God is still asking people to go and do different things in different places; are we ready to say "Here I am, send me"?