There is no read "border". The sun's particle and wind environment carves out an area called the heliosheath, but the Kuiper Belt extends far beyond that. Voyager 1 and 2 are there now, 93 and 114 AU from the sun.
"On June 28, 2010, Voyager 2 completed 12,000 days of continuous operations since its launch on August 20, 1977. For nearly 33 years, the venerable spacecraft has been returning unprecedented data about the giant outer planets, the properties of the solar wind between and beyond the planets and the interaction of the solar wind with interstellar winds in the heliosheath. Having traveled more than 21 billion kilometers on its winding path through the planets toward interstellar space, the spacecraft is now nearly 14 billion kilometers from the sun. Traveling at the speed of light, a signal from the ground takes about 12.8 hours to reach the spacecraft.
Voyager 1 reached this milestone on July 13 after having traveled more than 22 billion kilometers. Voyager 1 is currently more than 17 billion kilometers from the Sun."
The sun's gravitational influence extends to the edge of the Oort Cloud, gravitationally halfway to the nearest star in any direction, 50,000 to 100,000 AU from the sun. We will certainly not visit there in my lifetime...
MW