You have two options:
// option 1
PointerBuffer params = BufferUtils.createPointerBuffer(1);
glGetBufferPointerv(GL_ARRAY_BUFFER, GL_BUFFER_MAP_POINTER, params);
ByteBuffer buffer = params.getByteBuffer(0, size);
// option 2 (more efficient)
ByteBuffer buffer = MemoryUtil.memByteBuffer(glGetBufferPointer(GL_ARRAY_BUFFER, GL_BUFFER_MAP_POINTER), size);