Sorry it took so long for me to get back to you, things have been crazy here!
With Koi its kinda like goldies, trial and error with the different fruits and vegies. Each has a individual personality as well so some can be finicky or picky eaters while others will eat anything you toss out. However, its just getting them to take a taste of some things. Offer them more then once, give em a chance to get a taste or two.
Both stores are online, Dr.s Foster & Smith, and PSKoi, You have to email Russ & Kari for current prices and orders. Nice folks there.

F&S can send a catalog out also. I usually get both for AQ's and Ponds.
Well, as for the fruits, I couldn't get them to touch the fresh fruits at first, but started giving them the fruit treats and started slipping in real blueberry's as I have blueberry bushes in the yard, so then I slipped in real oranges pieces on them. Pineapple, watermelon, Kiwi fruits are some other folks have gotten theirs to eat.
Zuchinni and Yellow squash, green beans and peas are favorites.Spinach, leaf lettuces (not iceburg).. Cauliflower and broccoli are loved as well. You can also get some plain knox gelatin and make a vegie gel food. Shrimp, sardines, tuna any of the fish is good for them also. I make my own gel food for the goldies so the koi get gel food treats once in awhile. They go nuts for that.
I don't call myself a expert though, I believe we learn more and new things everyday, so a student would be a apt description, LOL!