Where can I find a replacement for this 3.5" door knob spindle?
If you're in an area with older homes, try your local locksmith.

The last time I had to replace a split spindle (where the split was not in the middle), the local lockmith had 'em in steel and brass.  I went with the brass, because I had to grind out a channel so it'd fit into the existing mechanism; you might have to do similar and drill it yourself.

The ones you see online probably aren't a press fit -- most of my knobs take a set screw.

You can also try a real hardware store (not a Lowes or Home Depot ... either an independant one, or an Ace, True Value, etc.)

If you strike out completely, I've also replaced the whole knob and spindle with a replacement set, and switched to a threaded spindle.  It looks like Van Dykes has a few :


http://www.vandykes.com/product/2-glass-knobs-with-spindle
http://www.vandykes.com/product/glass-knob-and-spindle-set
http://www.vandykes.com/product/glass-door-knobs-with-spindle


It looks like what you're looking for is referred to as a 'drilled spindle' (possibly drilled &amp; tapped, if there's threading inside the holes)

Using the search "drilled door spindle", I've managed to find a shop in the UK:


http://www.doorfurnituredirect.co.uk/scpro/bernards/productscategory~prodtype~58~prodtypename~Spindles.htm


... but many of the hits talk about pre-drilled doors, rather than specificall drilled spindles.