It is no practical problem to use a 75mm diameter instead of 80mm. It could even be that you have the correct size already, as (for the loudspeaker as such) it is the inner diameter that is relevant.
In case you were talking about the inner diameter and you want to obtain a "super accurate" result :-) , the relative reduction of area (75²/80²=0.88) leads to a longer tube. E.g., a 80mm tube of 220mm length can be replaced with a 75mm tube of 220mm / 0.88 = 250mm.
However, a +-10% change in area or length is not critical.
You could purchase another 75mm tube and cut it to the alternative length, just to try and learn whether you would hear a difference.
And you could also replace the bass reflex tube with a channel made of the same material as the enclosure. It simply must have the same area (80²=6400mm²) and length.