You can shop boots and gloves at either Gheed in act 1 nightmare or Larzuk in Act 5 (I think Normal) that will give both resists and MF. You just need to go to a waypoint or out of town and back to refresh the stock.
Other than that, I relied on a 34% mf amulet that I found. Just pick up every blue amulet that drops - one will be an mf amulet eventually.
Run nightmare Andariel until you get 2 nagel rings. They are very common.
That will put you over 100%.
Eventually, you’ll find items that give you more MF. Like I actually found a two-handed staff that gave MF per character level, so I actually switched to that when killing a boss. It also had +2 to all skills, so it was actually a pretty useful stepping stone item. I think I got that from Nightmare Mephisto or Diablo.
Generally, just keep running nightmare. I eventually found MF boots and gloves with 2 resistances on both, which really helped get out of negative resists in hell.
Also, I stumbled on a tancred amulet and boots, and that combination gives 78% mf so it’s another way of getting a lot of mf from those slots. It’s not that rare - I have found two copies of each actually.
With my current setup, I have ~250% mf with excellent resists after I found a 38% mf gheeds. It only took a few days.