Hi Andreas,
I just dropped in to Global Pet Foods, 2100 Bloor Street West (N side of Bloor between High Park & Runnymede subway stations on the Bloor line).
They have Nyger seed, white millet seed, & other (A helpful sales clerk told me, but I wan't paying attention.). Also bird feeders for Nyger seed, suet feeders, hummingbird feeders, & others. Some of the packages looked a little large, (not huge, just a little large) so consider bringing a backpack or plan to work your upper arms on the subway.
Half the store used to be devoted to birds, but I guess we dog owners (& some cat owners, but I won't mention cat owners on a birding site) have taken over most of their territory. There's a possibility this is the Birder's Nature Store you referred to.
Ph 416-604-9272 if you want to call before you set out. Open 9-9 except Saturday 9-6 & Sunday 11-5. No web site listed.
Hope this helps,
Leslie