Xanathar's Guide has the rituals Ceremony and Skywrite.
Remember to consider not just which rituals are best, but which rituals are rarest. Find Familiar is terrific, but it's also very common among wizards, and it's easy to identify which wizards have it (by the animal on their shoulder). So even if you don't take it right away you'll probably find it real quick. If you're patient you might be better off taking something that never appears in spellbooks -- like Purify Food and Drink.
There are a number of possible sources of written spells. Spellbooks and scrolls are the most obvious. But there are also ritual books -- many folks other than wizards take the Ritual Caster feat. Plus you can copy from other tomelocks (or make trades).
Then there are the non-magical sources. First of all, I find it preposterous to think that clerics don't write down their spells. Pretty much every temple everywhere should have some written guide, maybe even a bible of sorts, about how their religious ceremonies should be performed. It may be reserved for the inner circle of priests, or well guarded, or carved on temple walls instead of in a book, but there should be many in existence.
Also, a ritual doesn't have to be written down in a magical way like a magic scroll, ready to be cast with the energy stored in the scroll. Anyone capable of casting a ritual should be able to whip out a pen and start writing instructions on any old scrap of parchment. Don't even need expensive inks. If that caster is willing to cooperate and explain to the warlock what each step means, that should be good enough. This could be especially useful if you have a druid or a sorcerer in the group -- they could just straight up teach you the rituals they know.
