Well, there are some examples. As computer game player (not PnP) I know Fall from Grace (PST) and Zajive (NWN2) as clerics without god.
In the third edition, there was an ur-priest prestige class that had the ability to steal spells from deities. Though in actuality, ur-priests unknowingly received their spells from deities. Evil deities, usually. Evil deities who used the ur-priests as their pawns.
Ember from WotR is a witch, but she is quite similar.
In general, witches are more of an analogue of warlocks in Pathfinder. Well, you know, patrons, hexes, dark powers and all that stuff.
Usually all clerics and paladins need a god for their powers.
Not really.
Clerics who worship Archdevils and Demon Lords also gain cleric powers.
Moreover, in the current edition, clerics can derive their powers from Pantheon, Philosophy, or Force worship.
From Xanathar's Guide to Everything
The typical cleric is an ordained servant of a particular god and chooses a Divine Domain associated with that deity. The cleric's magic flows from the god or the god's sacred realm, and often the cleric bears a holy symbol that represents that divinity. Some clerics, especially in a world like Eberron, serve a whole pantheon, rather than a single deity. In certain campaigns, a cleric might instead serve a cosmic force, such as life or death, or a philosophy or concept, such as love, peace, or one of the nine alignments. Chapter 1 of the Dungeon Master's Guide explores options like these, in the section "Gods of Your World." Talk with your DM about the divine options available in your campaign, whether they're gods, pantheons, philosophies, or cosmic forces. Whatever being or thing your cleric ends up serving, choose a Divine Domain that is appropriate for it, and if it doesn't have a holy symbol, work with your DM to design one. The cleric's class features often refer to your deity. If you are devoted to a pantheon, cosmic force, or philosophy, your cleric features still work for you as written. Think of the references to a god as references to the divine thing you serve that gives you your magic.
