if the branches you find outside *snap* when you break them and you take the bark off they should be fine. soak them in a bucket for a few weeks, with weekly waterchanges, until they brown in the water is gone. i dont really bother to do this as the stained water looks great and the fish like it.
also, trees with needles (evergreens) are typically not as "aquarium safe" as hardwoods which loose their leaves. but if it is an old dry piece will be fine. the reason they are not cosidered as "good" is that evergreens have more sap which can leech into your water. but if the wood is "dry" or "seasoned" it should be fine. as mentioned, along streams or rivers is a great place to find wood.
cheers