I find that using these brushes can be disheartening if you don’t know how to bring life back into their uniform color. To add a little color, you can use the lasso tool to select the area you want a different color, then go to Image-->Adjustments-->Hue/Saturation. You can also play with the levels of each section to get darker or brighter colors. Other easy ways is to layer brushes on top of each other and chose the Color Burn or Color Doge options to experiment with the effects.
Here is a list of the websites that you can find these brushes. Many are found on Deviantart, again remember they are all free.
“Autumn Leaves” can be found at photoshopfreebrushes.com
"Harvest Brushes" , "Autumn Clusters" , "Leaves Brushes Megapack", "Thanksgiving III", "My Thanks", and "Thanksgiving Borders" can be found on deviantart.com.
Thanksgiving set can be found on the Official Adobe Photoshop site.
Vector Autumn Banners can be downloaded at Lordofedesign.com
This article provides even more Thanksgiving Photosop brushes.