best toner for blonde hair?

i have very light, level 10 blonde hair, but it has a little bit of a yellow tone to it. i'd like it to be whiter. what's the best professional toner in your opinion? i'm a cosmetology student so i can buy professional products. i've tried a couple different ones and it seems that they last for a few days, then the yellow tone comes back. i'm also not really comfortable bleaching my hair more as it's pretty damaged in some spots and i'm trying to get it healthier.