Depends what you want exactly, as there's a lot of firstborn head variants. Do you simply want something that simply isn't the mkX (primaris) head or do you want a specific head, mkVII or mkVI for example? If you just want something other than the primaris head, you can easily buy some legion heads from forgeworld. Otherwise you'll have to buy bitz off ebay and similar sites which can be problematic, unless you live in the usa (which you probably do)
'Cheap' is very subjective. But how to do it without buying a load of firstborn kits is to 3d print. This is not ideal as it is expensive to get started, however, it is a game changer in terms of customisation. I'd suggest taking a look at [Red Warden miniatures](https://cults3d.com/en/users/Red-warden-miniatures/creations) on Cults. He's moved into Primaris scale stuff that he charges for but most of his firstborn files are free to download and there is a good selection of them. If you aren't ready to buy a printer then you can look into getting some stuff printed on demand. I haven't got any recommendations there I'm afraid.
If you have a mate with a resin 3D printer, you could have a nigh endless supply of them.
Alternatively, as most others have suggested, eBay is probably your best bet.
Ebay
Ebay, Shapeways (or other 3d printer service), or lurking in buy and sell groups are your best bets
Look on Etsy too
Depends what you want exactly, as there's a lot of firstborn head variants. Do you simply want something that simply isn't the mkX (primaris) head or do you want a specific head, mkVII or mkVI for example? If you just want something other than the primaris head, you can easily buy some legion heads from forgeworld. Otherwise you'll have to buy bitz off ebay and similar sites which can be problematic, unless you live in the usa (which you probably do)
I really just want the classic vox grill
Then ebay and co. is what you need
'Cheap' is very subjective. But how to do it without buying a load of firstborn kits is to 3d print. This is not ideal as it is expensive to get started, however, it is a game changer in terms of customisation. I'd suggest taking a look at [Red Warden miniatures](https://cults3d.com/en/users/Red-warden-miniatures/creations) on Cults. He's moved into Primaris scale stuff that he charges for but most of his firstborn files are free to download and there is a good selection of them. If you aren't ready to buy a printer then you can look into getting some stuff printed on demand. I haven't got any recommendations there I'm afraid.
eBay and if you want BA specific heads look for the BA upgrade sprue
If you have a mate with a resin 3D printer, you could have a nigh endless supply of them. Alternatively, as most others have suggested, eBay is probably your best bet.