I am by no means an expert but I can share with you what I've done in the past and personal opinions. First, regarding store bought leaf litter, in my area all of the dedicated reptile stores (which excludes the big box animal stores) carry leaf little which is usually a standard freezer bag of magnolia leaves and or white oak leaves and they are sold for $10-$15 dollars and because I'm 98% certain that they are just collected from around the area and bagged, the price is just a bit too much. My opinion on magnolia leaves, unless it's a true dwarf variety, are too large for my taste. They will last a longer time than many other varieties of leaves but some live oak trees have small leaves and will last just as long.
As for collecting in the wild, I've done it many times, just made sure it was a park where sprays are used almost all the time. I take home what I've got and put them into a colander, give them a good rinse, then bake them in the over at about 200°F for 15 to 20 minutes. Probably not totally necessary, but a little caution can go a long way from keeping pests out. Plus most biology in a 7a zone is probably not going to servive in a tropical setting anyway. I'd also stay away from foliage that is aromatic as that is usually an indicator of oils in the leaves that are probably not good for sensitive amphibians. Examples would be like eucalyptus, and needle leaf trees, rosemary, lavender, etc...
The other thing that I do to get leaves is I have a couple planted bushes in my yard, pineapple guava (Feijoa sellowiana zone USDA 8-11), that give excellent leaf litter throughout the year thats perfect sized for smaller terrariums and is able to last decomposition quite the while along with some sweet edible flowers and unique tasty fruit that it produces. This means I can collect all the leaves I want and know exactly what's on the leaves or the the lack of, ie. pesticides or fertilizers.