Where can I find ads to place on my site that are
125px x 125px to place one my site? The sites content is mixed.
You can use Google AdPlanner to advertise slots you have available on your website. These advertised slots will appear for people on AdSense (mostly) and you can tell the AdPlanner to use any ad agencies you will find in the future to rotate the ads displayed in the slots.
It is not really finding you ads, if there is no one who is creating such ads ... but AdSense might auto-create text ads in that format in some cases.