Starter for a barrel

How big of a sour starter do I need for a 30 gallon barrel? Will a half gallon be enough or should I have a 1 or 2 gallon one?

It really depends on a bunch of things:

What style are you making? The only style you would want to ferment in a barrel is a lambic.
What strain(s) are you using?
What is your ferment schedule?
What is your OG?

This topic is pertinent to myself and a group of friends as well. Here's what we are doing, to give you an idea:

Nine of us are brewing 5-10 gallons each of a belgian style imperial stout with an OG of 1.085. The barrel is a former whiskey barrel and will be 55 gallons.

Our approach to souring will be that everyone is brewing it with Wyeast Ardennes yeast. We'll be filling the barrel completely with that and leaving it for a couple weeks. In the meantime, I'm doing a couple low gravity batches of beer with a Roeselare pitch. My 5 gallons of stout will be fermented on this 2nd or 3rd generation Roeselare cake.

After a couple weeks, we'll be pulling 5 gallons back out of the barrel, just to see what happened to it in a short amount of time, and replacing it with my Roeselare batch. This will act as our sour starter.

As mentioned above, you probably don't want to do your main fermentation in a barrel unless it is a lambic. One of our reasons for doing a single 5-gallon batch as a sour starter rather than souring all of them initially is simply the cost. This way everyone isn't investing in bugs. Just one of us.

It is going to be a lambic. The half gallon starter I have is an Al B blend, contains Russian River bugs and others. My OG will be around 1050.

Oh, I have a healthy slug of the same strain, in the fermenter of the best sour I ever made.

I really think a fresh step to a full gallon starter or perhaps the half gallon on a stirplate just to ensure your ferment takes off, but it is a lambic - so it may not be the end of the world.

I am not sure what stepping up the blend will be though. There are particular ratios of bugs to brett to sacch, so by doing the starter you are growing the brett and sacch faster than the bugs...

I am thinking about adding the half gallon starter, a smack pack of the wyeast lambic blend and 2-3 bottles of dregs. Think that will be enough?

Should be. You may want to wake up the smack pack and the dregs with a small starter too.