Listen to Chuck, don't go synthetic if at all possible. I don't go for the bagged commercial stuff myself, I hate spending money. My best tip is do you have anyone keeping horses near you? They shit a huge amount and people are usually very glad to find someone who will shovel it up and take it away. The only concern is whether they have been fed weed treated hay.
Poultry manure is excellent if you can find someone keeping chickens, it can be a bit strong, but add it to your compost heap.
I have seen people here who collect coffee grounds from coffee shops, any sort of organic waste like that is useful.
If you are concerned about poisons make a long term compost heap, the nutrients won't disappear, but things like caffeine and weed treatments will degrade given enough time.
When I make compost with stuff like weeds and hedge clippings I run it all through the mower first, it really speeds things up, not a lot of nutrient in that, but adding a bit of manure will help all round, and in quite a short time you can have an excellent mulch.
When I take plants out I run over the area with a rake, a lot of the mulch will have disappeared into the soil, but there is plenty of bits of twig and tougher stuff, I sling those back into the compost. When you add stuff like that you are also adding lots of the micro organisms that break things down, a sprinkle of soil occasionally is good too.
The only organic material route I wouldn't go nowadays is adding cardboard and paper to compost or mulch generally, so much of it is made with recycled stuff and has plastic added to hold it together now. I do use cardboard boxes flattened to kill off weeds in places that are completely unkempt, but they will come up in time, I won't dig them in with the dead weeds.
If you burn stuff, or have a wood stove, the ash is a good source for potash.
