Great question, i think looking for a specialist or an agent that has a niche market is a great idea if you are sure that that niche is the way to go. The hard part is that if you are buying homes for investment it would be wise to keep all options open. All the deals left out there aren't necessarily only going to be short sales or probate sales. Although i agree that in my opinion there is tremendous value and reasons to look into short sales, but without proper understanding of you personal goals it would be hard to make sure that it is a fit.
Now Finding experts, well just like anything else interview them, testimonials, etc. There is no magic answer its all about doing the research like you are now and staying consistent. You might find a great agent, or you might work with a couple of agents. The best thing to do is be honest with them if you are working with other agents for other types of purchases.
You might also want to ask your self is a buying agent really an expert on short sales, although many agents have experience and have put offers and closed short sales. There aren't that many buying agents that actually are out there consistently obtaining short sale listings. Short sale listing agents in my opinion are a different breed, it takes a specialized agent to keep acquiring short sale listings and be good at closing them. I would say as well finding a specialist when it comes to probate will be tough. There are many experienced in one transaction or have the knowledge to do this, but this is a pretty small market to build your business on.
I also agree with your statement, yes short sales are indeed closing i can vouch for that.
Good Luck and any help you need feel free to contact us.