Summoning by Level
Here are some suggestions for how to progress from Level to Level

Level 0: Rabbits

Level 3: Rats - Rats are plentiful, and provide all the reagents needed to summon them (except the life essence of course)

Level 6: Racoon - Raccoons are the best resource usage for the low level summoner, so plan on hunting and summoning lots and lots of them. You'll summon racooons for a long time.

Level 15: Snakes - About here you can start summoning snakes, stick with green or red mostly as they require less meat. Probably best really to stay on raccoons.

Level 22: Foxes - Again Raccoons are better resource usage, as you will get more xp per life essence and you'll have to hunt something besides the foxes for meat.

Level 26: Boars - You will have to have a lot of meat, but you don't need furs so anywhere you can get your meat is great.

Any other idea's posting your suggestions will greatly help this topic. Enjoy!

To stock up on bones and snake skins, you could check out Naralik. It now has a storage.

There are goblins there, so don't go there unless you fight them. These goblins drop 2 bones and usually some gc when you kill them.

There are alot of snakes in Naralik too.

There is a hazardous storage npc by one of the houses in the south east corner.

Happy hunting.

If you are serious about summoning, you want to follow the summoning god, it's in WS.
I would advise against the summoning perks and extra charm until you can summon the big monsters.
You will need alot of snakes and alot of meat.
I would bypass the foxes and stick with snakes as long as your sanity lasts, like nathan says, Naralik is a great place to gather snakes.
Summon boars until you get into the 30s, then switch to wolves.

good luck