Your component-supply problem in NZ is perhaps a little worse than ours in VK but not much.
You have, as I have, Jaycar and thank goodness for them; they are great for basic parts like resistors, cases etc but nothing terribly exotic.
I bought my "stash" of the MRFs from ebay which is where I get quite a lot of "exotic" things that our US brethren can get from Mouser etc.
W8DIZ is a great source of ferrites and far cheaper than I can get them here; in fact, I'm about to check my stock and order some from him. He charges minimal postage.
Mini Kits in VK5 is a good source of some things, good connectors for one. Not cheap but Mark VK5EME sells good products at a modest profit and you can't ask more than that.
Allison is correct (as always
) about the ferrite type; 43 is probably the all-purpose material for HF/low VHF with 61 useful to get up a bit higher.
I suspect that the roll-off in my amplifier is due to the transformers running out of steam; the peak may also be due to resonance in the transformers.
PM me and I will give you my e-mail address if you want to have a "QSO".