Despite being a Realtor, I would highly suggest going to the seller themself. I find that most realtors are not as well educated as they should be and can many times interfere with the deal at pivotal moments.
This is especially true because if there's a realtor involved, the property is likely to be listed and available to anyone. In this situation, there's a good chance you'll be circumvented and lose the deal. I don't want to rip on my own kind, but this frequently is the truth.
If you're not direct to the seller, they're all likely to be going around you. Being direct to many sellers, I find buyers going around wholesalers all day long.
As for using a realtor, a lot of the advantages to using them depend on their knowledge and understanding of investment real estate. Most of us will tell you we know, but most of us definitely do not. Learn to ask key questions that will expose their lack of knowledge quickly.
If they look like a duck and sound like one, they're likely to crap like one too...pick someone who knows what in the heck they're talking about and can prove it through good solid information.
Good luck!