That all depends on the case, and where/how the fans are mounted. Just be advised that the 80mm fans will be noisier than the 120mm fans that push the same amount of air (CFM). I'd personaly just use the 120mm fans (they cost maybe $2 more).
Fitting fans is not rocket science. You can always use zip ties to mount a smaller fan. If the case says 120mm fans, it means that there are pre-drilled mounting holes for 120mm fans.
That said, you do NOT want to use 80mm fans if there is a place for 120mm fans. A 80mm fan has 45% of the area of a 120mm fan, and will not cool nearly as well. They will spin much faster and be much noisier. At $7 for yate loons mentioned by IH8U, they are a no-brainer.
Some cases have pre-drilled holes for multiple fan sizes. For maximum airflow and minimum noise, use the largest ones you can.
If you are limited by budget (maybe there's none), but happen to have some 80mm fans lying around, then you can use the zip ties geofelt mentioned to mount them where a 120mm fan would fit.