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 Horizon360,Roon/HQP, Nvidia SHIELD, Trinnov Altitude16
Electronics-DoshiPre,Cary2A3monoamps,512 Engineering amp
Speakers-AvantgardeDuosLR,3SolosC,LR,RR, ML ceiling and side speakers, Velodyne DD18 sub
Other-Scott Rust Interconnects,2x512Engineering-Marutani Power Supply
Music-2300 15ips2trackR2Rtapes, 500 7.5ips tapes, 1.5KCD's,500SACDs,60TBripped files