The responses to this is based on individual's experiences.
Then again, you can attempt to use math and probability to work out the chances of obtaining a spell whether through farming the pack or Loremaster.
But, I must say, spells from packs seems to exceed the rarity of Epic. I'd maybe call it 'Legend.'
You may have better chances farming Loremaster.
What I can also say is that certain Holiday Spells such as Headless Horseman is readily/easily available from The Loremaster as she seems to drop spells like these at a higher % rate during Halloween (in this case) as opposed to any other time.
When trying to get spells from packs is it the same chance as with the loremaster? Or is the chance for each slot in the pack?
I believe it is a chance for each slot but the change is a very low percentage rate. A lot of people choose to farm Lore because she drops them without costing real money, but the down side to this is the time it takes and also the fact she has so many spells that even if you get one it might not be the one you hope for.
I always feel quite lucky when it comes to packs but I think it is because I never open them with just one specific top tier item in mind. I know there is the chance I'll get them but I always want other more common drops from them too. I feel lucky because I got that cute binny jezzerite housing item or a baboon thug...if you're only going for the rarest drop sometimes these nice things go unnoticed or appreciated.
I recommend if you go down the pack opening route that you check there are other things in the pack you'll also feel good about getting and that you keep in mind opening packs is giving you a chance of the spell, not a guarantee. Set a budget a few packs below your limit before you start...then you can still do an extra one or two 'for luck' lol. You'll feel good about what you get and if you don't get the spell in your budget you will feel more inclined to head back over to Loremaster for a few runs.
Be kind to yourself by throwing in a bit of realistic thought to your optimism...the spells DO drop from packs, it is impossible to know whether you will get it this pack or next or this run of Loremaster or next...
Don't forget you can also craft the spells too...planting some king parsley for amber drops might be worth doing before you actually need it
Whichever route you take, I wish you a super big double dose of wizarding luck! I hope you get your spells soon!