terrific reels, the Cardinal 3 has a tremendous following and they sell for more than the original price..
Cardinal 4s unfortunately aren't worth as much, though there are some very ambitious pricings on ebay..
I was looking for a 3 for a while until I realized they were all going to be in the hundreds, more than I'm willing to pay. The 4s seemed to sell at lower prices, when I was watching earlier this year.
Spools are readily available there too, though pricey.
According to Mikesreelrepair dot com, part number for the spool is 11526, can also use -- 970515, 11526, 1047274. None in stock unfortunately.
A trot through the webs googling for these numbers and 'spool' might turn up something.
That small button spool looks like an original, though. Here's a 1970 ad which shows that same spool -
I'm a Mitchell enthusiast, even I agree with them the Cardinal 4 is a better reel than the 300..
But the 3 isn't better than the Mitchell 308 with upgraded line roller ;-)