The April edition is on the table next to Squall’s bed in his dorm room and that should still be there if you go back. You’re not going to be able to get back to the Deling sewers any more, but if you do need that magazine there’s a shop on disc 3 where you can buy it from.
There’s only one more on disc 2 to find, you’ll end up visiting Trabia Garden at some point and the August edition is in the third (I think…) screen, you’ll see a fountain in the middle and if you walk slowly back towards you, the magazine is on the floor very close to the screen exit.
The July issue appears in the left-hand path in the training center but only on disc 3, then there is one more that appears later in disc 3 but I think that one’s very obvious to find.
You can actually upgrade weapons without reading the magazines – as long as you have the items you need, the upgraded weapons will appear at the junk shop. If you learn the card mod ability and refine some of your low level cards you don’t need into items, you’ll probably find you have enough stuff to upgrade some weapons straight away (and maybe learn a few new blue magics for Quistis while you’re at it).
While it’s good to do the weapon upgrades when you can, the strength boost you get from that isn’t huge. Your junctions will have a bigger effect overall so it’s good to look at those too.