One note of caution, in this country there is no clear definition of top soil if you are buying it. It could be good organic rich top soil or it might not be. Ideally you would like something like sandy loam, you might however be sold clay which is very hard to amend. It is also possible to simply fill the boxes with mushroom compost if you can buy it by the yard. It will then have to be added to every year as it will continue to compost down, but you would want to add compost yearly anyway. That is what I finally used in my carrot boxes, because I need something that did not have rocks and wasn't going to lump up. That was about 10 years ago and all 3 are still growing great crops (3 for rotation purposes). Whatever you choose, it will be cheaper by the yard by far than by the bag.