My Janome is very picky too, can't use regular class 15's; they HAVE to be Janome brand or I have problems.
I buy them on Amazon but they're not cheap. I have 2 Janomes though (my main machine + my class machine) so I've slowly accumulated a good supply. I'd love to buy them from my dealer but they want about 150% the price they are on Amazon....ouch!
I have an older (1990's) Kenmore that was made by Janome...THAT machine *will* handle regular class 15's and I've even gotten it to work with class 66's, but it's a vertical bobbin, not a drop-in like my other 2. I think drop-in bobbin machines are more sensitive to having exactly the right bobbin.