You have a couple of options: contact the seller yourself, contact the listing agent who had the home listed, work with your current agent (if you're working with one) or contact an agent who you've worked with in the past or have gotten referred to.
There are pros and cons with each of these approaches. If you work directly with the seller, you both need to be aware of all the paperwork and processes necessary to purchase a home, including any city inspections, disclosures, etc. which can be tricky. The pro is that you might be able to negotiate a great price for the home.
If you contact the listing agent, please remember that the agent will not be representing you without a Representation Agreement. Most likely, they will be retained to represent the seller or act as a facilitator. In either case, you will not have the benefit of an experienced agent in the negotiation process. The advantage of this scenario is that you will have someone who understands and lead you through the process but you may not be able to negotiate as low a price.
If you chose to use an agent that represents you, you will have someone that is looking out for your interests and who can negotiate the contracts for you and ensure that the purchase makes it to closing as smoothly as possible.
If you need any further information, please feel free to contact me.
Yes, one of your options is to contact the Seller direct. Yet another option is to contact the last listing agent and work out an arrangement. Sometimes a great relationship still exists between the seller and their past agent yet, so put it to work for you and your buyer to create a sale. I've had good success with this.