Hi Rachel,
I hope I'm understanding your question correctly; it sounds like you're looking for a replacement space bar, what is known as a key cap, is that right?
There are definitely places you can buy those; personally I gravitate toward eBay; a quick check shows several sellers offering to sell key caps from your keyboard. The keys for sale there seem to be running anywhere from about $2.50 to $5.00 US.
If you're looking for something a little more professional, there's a site called [link|https://www.replacementlaptopkeys.com/apple-imac-wired-a1243-aluminum-with-nppad-keyboard-key/|Replacement Laptop Keys|new_window=true] selling them but they have a caution that there are two different key styles used on that keyboard and they're not interchangeable, so you have to be sure you're ordering the right ones. Most of the eBay ads call them Type F and Type H, while the Replacement Laptop Keys site calls them type AC05 and AC28. RLK site is asking $4.99 for your spacebar, I assume there's also a shipping charge, which should be minor.
Not all eBay sellers specify which key type they're selling (they may not even know there's a difference), so unless they show a picture of the key's hinge mechanism so you can tell if it matches yours, you'll want to avoid those.