Probably one of the first sourcebooks put out for Advanced Dungeons & Dragons was Deities and Demigods, a book with a collection of beings that would provide something for your Cleric to be, well, a cleric of. However, it doesn’t really hold up very well, particularly compared to later deity books for later editions of AD&D and D&D.
The problem with the book lies with the fact that the book is very much at odds with itself. The first chapter or so of the book talks about the nature of deities, how they can be used in game, and admonitions about using the book as a de-facto monster manual to sic your players on. And then, unfortunately, the rest of the book’s descriptions of gods are written up in the same format as the Monster Manual and Fiend Folio, with the deities written up using the same stat blocks, and descriptions focused entirely on physical appearance and their battle tactics.
This is a shame, because later editions of AD&D and D&D would get much more in depth of what effects deities have on player characters, in terms of their agendas, and in terms of what it means to worship them. We don’t get information on in what ways gods prefer to be worshiped (and how frequently). If my cleric of Tyr prays to his deity before every battle, will that actually get his favor (which will help my party in battle), or will that annoy the crap out of him – or is praying before every battle the default state and I will be penalized if I don’t seek his favor?
By comparison, later editions of D&D put a lot less focus on the idea of the Heavens (and Hells) as the ultimate epic level dungeons, and instead put more focus on the hows and whys of worship – why would you be a cleric of this god, and how do you show reverence to this being? In particular how do you properly worship this deity in a manner that fits with the adventurer’s lifestyle?
This is especially the case with the introduction of spell spheres in AD&D 2nd edition – a little bit of added crunch that tags spells with keywords that in turn are attached to various deities. Clerics who worship a particular deity get (depending on the edition and GM) a spell list related to those spheres that you are either limited to or which provided additional spells you can choose from.
The 1st Edition version of Deities and Demigods, on the other hand, leaves players with significantly more grunt work to do… so much more grunt work that you’re almost better off not picking up a copy of the book and instead just researching the pantheon you want to incorporate in the game.
If you want to pick up the book, the revised edition (which excises the Cthulhu and Melnibonean mythoi) is available from DriveThruRPG and the Dungeons Masters’ Guild.