How can I spawn in an item I don't have for testing purposes?

Have you considered just buying or trading for one?

A Strange* Market Gardener hovers around $0.60 on the Steam Community Market.

Otherwise, you can trade item for item on sites like backpack.tf - most listings for the Market Gardener ask for two weapons/1 scrap metal in return (sometimes listed as '0.11 ref').

_{* 'Strange' weapons track kills but are otherwise identical to regular weapons}

There are 2 main ways you could go about doing this.

Borrow an item from the Mann Co Store

1. From the TF2 main menu, click on "Shop"
2. Go to the "Items" section
3. Search for/find the item you want
4. Click on the item, then click "Try it out"

You will then have the item for free for 1 week. It can be used in all servers, however you can only do this with one item at a time.

Use the itemtest menu

1. Open any map
2. In the console, type itemtest
3. Once the itemtest menu opens, check the box of the class you want to test weapons for (in this case, Soldier) There should now be a box, among others, labelled "Weapon."
4. Click "Add" next to the weapon box
5. Select the item you want from the dropdown menu, then press "OK"
6. Click "Apply Test Items"

One thing to note with itemtest is that it will not work properly with bots.
This will only work on the map you're currently on.