You can buy just a toilet tank lid and the cost of the toilet tank lid ranges from $39.00 to $80.00 depending on the brand of toilet.
Lowes and other hardware stores sell just a toilet tank replacement lid and you can also order a replacement toilet tank lid online.
You can also buy a toilet tank itself although not just any toilet tank will fit any toilet bowl.
Some toilet tanks and toilet bowls are interchangeable but not all of the toilet tanks are.
To find a compatible toilet tank for your toilet bowl you should match the specific model and brand and also pay attention to the size and alignment of the bolt and connection holes as well as the flapper valve diameter to ensure that there's a proper seal and function.
Different brands and manufacturers of toilets use different toilet tanks and toilet bowls.
If you can't find the correct toilet tank for your toilet bowl you will need to replace the entire toilet.
Replacing the toilet tank is possible but replacing the entire toilet is usually best and sometimes cheaper in the long run.
Also high efficiency toilets and low volume toilets sometimes have different sized flapper valves, which also require that the toilet tank has a matching opening size.
And the overall dimensions of the toilet tank and toilet bowl also must be compatible.
Using a toilet tank that is too large or too small can cause the seat to also rest improperly and lead to other functional issues.
You can replace the toilet tank itself without replacing the toilet.
Although you'll need to make sure the toilet tank has the correct hole placement, 2,3,or 4 holes for bolts (4 is uncommon).
People usually opt for replacing the toilet however.