I have used the above search engine. I have personally bought ammo online from:
unammo.com
outdoorlimited.com
As mentioned, shipping can be expensive. All ammo has to go by ground shipping. Most online sellers use UPS or FedEx as their shippers of choice. Some sellers have a flat rate shipping policy, others get your zip code and then quote you a shipping cost. I tend to wait until there is a sale on the ammo I want and then purchase it. I have not seen anyone offering a lot of 10K 9mm ammo for sale. Those days might be gone.
I see that you live in the great state of Wisconsin as do I. There is Wisconsin Cartridge company near Friendship, WI that sells at most in state gun shows and also will do in person sales with prior arrangement at their place near Friendship. You will have to pay the WI sales tax but avoid the shipping charges. I still don't know about buying that many rounds in bulk. You could always call and find out. I have personally shot his 9mm ammo in a variety of my 9mm handguns and it runs just fine.