We've gotten spurred bits from
www.ehardwicks.comPhone is (206) 632-1203.
Not sure if they are Irwin - but they have had double spur, single flute bits. Some have an end for a hand brace, others for a drill chuck.
I see they are upgrading their website... but they have very knowledgeable sales folks and carry all sorts of oddball stuff.
Lehman's also has spurred bits.
I have a 1 1/2" bit from them... broke the 2" bit and returned it.
http://tinyurl.com/dg55ms