What are some third party sources for spells.

[Throne12]

I created a magic item and gave it to my party. It a key that gives a person spells and spell slots of a caster of there level. So when they put the key in back of there neck and turn it they have to roll a d6 to determine which spell list they get. So I want spells to us out side of wotc stuff to use. To Represent that this key was made with ancient magic that's no longer used and it knowledge is lost.

[nickl_2000]

Elminster’s Guide to Magic and Midgard Heroes Handbook from Kobold Press are both pretty decent.

[MrStabby]

Happy to drop some of mine in, although the appropriate place might be the homebrew forum rather than here.

What kind of power level are you wanting? Ancient spells from before people got good at magic? Spells forgotten because they were ineffective?

A lot of my spells I made were designed as loot, so specifically designed to be a power up for those that find them.

[Galithar]

I have greatly enjoyed The Blackstaff's Book of 1000 Spells (https://www.dmsguild.com/m/product/302120). Not wvery spell is balanced, but there are a lot of good ideas. Many can be used straight from the box, some need a little tweaking for balance.

[Mutazoia]

I would add a word of caution here. Spells from outside the system can be potentially game-breaking. Even spells from previous editions can wreak havoc on your game. Make sure you take a long, hard look at any 3rd party spell before you introduce it into your game. Try to think of how you would abuse it if you were playing and the DM gave it to you. Be assured that your players will come up with a dozen more ways to abuse it than you will.

[Galithar]

I would add a word of caution here. Spells from outside the system can be potentially game-breaking. Even spells from previous editions can wreak havoc on your game. Make sure you take a long, hard look at any 3rd party spell before you introduce it into your game. Try to think of how you would abuse it if you were playing and the DM gave it to you. Be assured that your players will come up with a dozen more ways to abuse it than you will.

Honestly as long as the Spell roughly follows 5e design and isn't glaringly broken I don't think this is a huge concern in 5e. Even spells that are "broken" don't get THAT ridiculous in 5e. Again, assuming a basic following of design intent, which I understand not all 3rd party sources will.

As an add-on to your point though, as a precaution, whenever I add anything that isn't official I make it clear to my players that it is subject to change or removal at any time if I feel it is getting out of hand. I've never had to use it on a spell, it's usually things like classes that are HomeBrew that I missed an overpowered ability on. But that's a whole nother mess.

[thorr-kan]

Elminster’s Guide to Magic and Midgard Heroes Handbook from Kobold Press are both pretty decent.
The Deep Magic Kickstarter from Kobold Press is hitting the delivery phase shortly. As a patron and contributor to the PF edition, I think it will greatly expand your game.

[Throne12]

The way I have it set up in my game world is today magic is standard practice through out the know world to the pc. But back in the age of Dragons there where all different ways and sources of magic. Today's magic is fueled by the soul of the dragons that is lingering where they died which is pretty much everywhere now.