Well to start, there are two types of manure:
A) Green Manures (which are just plants) and they include: i) Nitrogen fixers ii) Non Nitrogen fixers
Examples of N fixers are Clover, vetch, legumes (lie peas and beans), elaegnus mulitflora (Goumi) and so on
Examples on non N fixers are: Rye, wheat, barley and so on.
B) Animal Manures
Cow: Rich in Nitrogen, contains no seeds
pig: Also Rich in Nitrogen, again no seeds
Horse: A bit lower in N, contains seeds
sheep: High in Nitrogen (needs to be composted before adding directly to plants), no seeds
Chicken: High in Nitrogen, no seeds
Mushroom manure; Is a combination of I think cow and chicken manure and is very good.
Where to get manure, well the most expensive source (and easiest to find) is at your local nursery or (I hat to say this) at your big box store.
Local farms are a cheap (if not free) source of manure.
Try local chicken farms as well. I live next to a place that has 10 000 chickens and they sell the composted chicken manure for 3 dollars a 50 lb bag.
I get horse manure from local farms for 50 cents a bag.
If bought from a nursery or what have you the going rate is about 2.50 a bag.
C) Might as well include this while talking about manures.
If you live near an ocean, seaweed (especially kelp) is a free and wonderful source of a so called green manure.
If not, you can by kelp meal and liquid seaweed fertilizer at nurseries
NOW, all this being said:manures are what we call greens. If you are gardening the point is to not just put greens in your beds. If you do not only will you not be adding structure to your soil you will also be adding to much N to the soil. So, you want to add what we call a BROWN which is a compound that is relatively rich in Carbon. Good browns to add are leaves, straw, shredded black and white newspaper, cocoa bean hulls and so on.
Feed the soil, not the plants.