@Peter Shin
Getting an agent/broker to understand sub-to or a wrap will be very difficult. They will generally talk the seller out of it regardless of whether the seller has other viable options. Anything an agent/broker doesn't understand will be considered illegal, immoral or unethical in their minds.
The agent is generally not authorized to provide a buyer with any confidential information on the seller or seller's financing. All you can do is submit the sub-to offer in writing to the agent and send a copy to the seller as well. Once you send the offer, you can follow up directly with the seller if you wish. There is nothing illegal about direct communication between buyers and sellers.
You will need a real estate specific Texas licensed attorney to draft the sub-to offer...unless you just want to broach the subject with the seller via a Letter of Intent.
In addition to the offer terms, a savvy seller will want you to prove your have ample ability to pay both notes (your existing home and the seller's note(s)). But a good real estate attorney will help you understand how to do that as well.