<div class="select-wrapper">
<div class="select-inner">
<select name="select" id="select">
<option value="1">Первый</option>
<option value="2">Второй</option>
<option value="3">Третий</option>
</select>
<ul class="dropdown-select">
<input type="text" placeholder="Найти" class="search" id="dropdown-search">
<div class="dropdown-block">
<li class="dropdown-select__item">Первый</li>
<li class="dropdown-select__item">Второй</li>
<li class="dropdown-select__item">Третий</li>
</div>
</ul>
</div>
</div>
let sel=$('.select-wrapper>#select');
if (sel.length>0){ // если структура уже обрамлена - сюда не попадем
sel
.wrap('<div class="select-inner"></div>' )
.after('<ul class="dropdown-select">'+
'<input type="text" placeholder="Найти" class="search" id="dropdown-search">'+
'<div class="dropdown-block">'+
'</div>'+
'</ul>');
let ddblock=$('.select-wrapper .dropdown-block');
$('option', sel).each(function(){
$('<li class="dropdown-select__item">'+$(this).text()+'</li>').appendTo(ddblock);
}
)
}