I know you can access them once you finish a guild, but is there anywhere else I can go? I know asking for work in a tavern will generate one, where else?

This is something I never full understood the meaning of.

a - Some define them as Random quests.

b - Some see them as never ending loops given by Jarls to kill Bandits.

c - Some see them as doing a questline and getting this place to go to. Then doing it again with another character and getting a different place to go. Thus making each playthrough seem a little different.

I am leaning towards c.

A, B, and C.

- When you speak to inn keepers can you ask them for work. They will hand out bounty letters where you have to find somebody and kill them. The reward is given by the Jarl. You can also ask the Jarl and/or their Stewards directly.

- The Winterhold College can give you quests to retrieve books and without a kill requirement.

- The Thieves Guild offers 6 or 7 different types of quests where you can do all sorts of things, all involving sneaking, breaking in, lock picking or pick pocketing.

I have not finished the Dark Brotherhood quest line, but I expect similar tasks with them.

When I finished the Dark Brotherhood questline, it looped back to the first quest you are given but the travel marker just refers you back to the DB hideout, then when you go into the hideout the marker points you outside. Either mine is glitched or there is no finish.

Without posting spoilers, there are radiant quests for the Dark Brotherhood questline after completion. Also, there are radiant quests for the Thieves' Guild but I don't know if they're unlimited like the DB's

You should get your contracts (After you finished the Redguard's ones) from the...

All quests given by guilds post their main questline are radiant, with the exception of some DB contracts, some special TG jobs and some high level CoW quests (trying to use spoiler neutral terms here).

All Jarl quests post thane, all bounties, all repeatable favours and some other quests which randomize dungeons are also radiant.

If it doesn't have a story it's likely radiant, same as if the NPC doesn't give any names.

wasn't there a radiant quest where you picked potatoes for farmers?

Not sure. Some farmers, miners, hunters and wood millers will buy your stuff. Some will say lines like "good pay for honest work". It may feel like stealing from them or like doing a quest to you, but it is what it is - paid work.