I think that the only way to buy ff1 and ff2 is together as ‘final fantasy origins’.

ff3 was never released on ps1 in english.

you can get ff4 on it’s own or you can get it with another game made by square (chrono trigger) in a package called ‘final fantasy chronicles’ or you can get it with ff5 which is called ‘final fantasy anthology’ i think.

You can get ff5 with ff4 as i mentioned before or you can get it with 6 which is also called ‘final fantasy anthology’ that may get confusing, always check which one your getting.

ff 7-9 you buy separately but on amazon their are deals you can get, i bought ff8 and ff9 together for $15.
Keep in mind that ff7 is the most expencive by FAR. The price varies a lot but you probably won’t be able to find it for less than $40.

This is how i suggest you buy them:

-final fantasy origins(1+2)- about $20 AUD
-Final fantasy chronicles(4+chrono trigger)- about $20 AUD
-final fantasy anthology(5+6)- about $20 AUD
-final fantasy 7- anywhere inbetween $40-$200
-Final fantasy 8- about $10-20
-Final fantasy 9- about $10-20