My go to place for tape, tape supplies, etc. is
http://usrecordingmedia.com/ in PA. Bought from them many times, enough to qualify to get one of their tee shirts!
For specifics about the nagra, you will get good advice from the Tape Project forum. www.tapeproject.com. Several Nagra owners there.
Larry
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Analog-VPI Classic3-3DArm,LyraSkala+MiyajimaZeromono,Herron VTPH2A,2xAmpexATR102+OtariMX5050B2+Merrill Trident Master
Dig Rip-Pyramix,IzotopeRX3Adv,MykerinosCard,PacMicroModel2
Dig Play-Lampi Horizon,NADACmch,Roon/HQP,Oppo105
Electronics-DoshiPre,CJ MET1mchPre,Cary2A3monoamps
Speakers-AvantgardeDuosLR,3SolosC,LR,RR
Other-Scott Rust Interconnects,2x512Engineering-MarutaniSymmetric Power Supply
Music-2000 15ips2trackR2Rtapes, 500 7.5ips tapes, 1.5KCD's,500SACDs,50TBripped files