any reason for not going with performas? occasionally colsons pop up on
https://www.surpluscenter.com/
grainger sells off brand colsons (albion brand). the durometer is the same as a colson performa.
most of the time I buy my colsons from robot marketplace, but i've bought some off surplus center too.
edit: you could also look into using two of the orange banebots wheels on the same axle. it's a 40a durometer wheel so it should get more grip than a 65a durometer colson. i wouldn't recommend the 30a green banebots wheels, they wear down like crazy. also don't worry about the hole on the banebots wheels not being centered. it wears down to a usable "roundness" after some use...