Boulder Bicycle has the QB-75 113mm BSC in stock for $46 + s/h, but their online orders are temporarily halted until 10/21. You can email them or call them to see if they’ll process your order.
I’ve purchased 2 QB-95’s from them and an assortment of parts for my vintage builds before. Their prices seem good and with the supply chain problems affecting everyone you may need to pay a little more or wait an indefinitely long time for the QB-55.
IRD QB-75