<form action="/ads/category/" method="get" class="search_block" id="search_form">
  	<fieldset class="no_border">
	<select id="category" name="category_id">
	<option value>Во всех категориях</option>
	<option value="1" class="opt-group">-- опция1 --</option><option value="9">опция2</option></select>
	<select id="region" name="location_id">
	<option value="1">регион1</option><option value="2">регион2</option><option value="3">регион3</option><option value="4">регион4</option></select>
	<input class="goto" type="submit" value="Перейти" class="search">
	</fieldset>
	</form>
	<br>

var link = 'ads/category/?category_id=1&location_id=1';
link = link.replace('?category_id=','');
link = link.replace('&location_id=','/');
console.log(link);