No, I don't keep spares for my Amigas. I find if I have too many cards around I will build an extra machine I do not need. I had a spare floppy but I installed it in my A2000. I have a GVP 030/40 card for my 2000 that got swapped for a 2091 SCSI card , when I get bored off the stock gaming setup the 030 will go back in and the 2nd floppy will come back out.
Now for Mac and PC I have tons of extra parts, then again they are cheaper to come by and to be honest there were just tons of companies supporting those platforms making more then just accelerators and video cards.