Try ebay. Search for Bertren, then email the seller. If they have a store, there's a better chance they can help you.
If not, go here:
http://www.nehobby.com/
email them (I don't know if they answer them, I never tried) or better yet, call them and ask for Bob Bell. He's been in the business for about 35 yrs. If he doesn't know, i'm sure he can point you in the right direction.
They sell Marklin and LGB, but I have no idea if they handle Bertren.
Hope this helps.