Most hardware stores aren't going to have a 5mm reamer and a numbered drill set is about as close as you can get with resonable expectations somebody might have one or be able to get one.
BTW, Harbor Freight (no matter how you feel about them) is good for a cheap source on something like a numbered drill set. Sure, they aren't the longest lasting bits, but numbered drill sets are not intended for everyday abuse-they are for drilling that precision hole when you need them, and this being aluminum, they will work forever.
Also, I must add that more than once (I have a lot of bots) even the correct pulleys were tight on the motor shaft due to the "crimp" which holds the top and bottom belt sheeves and ended up reaming them with a 3/16 (for the old motors) after I had removed the set screws.
These are $7 pulleys so "perfect fit" isn't to be expected.
Ok, and before we get too far into this, MBI gets the pulleys from SDP (same place I get mine), and SDP has been out (not even listing) the required 17tooth GT2 2mm belt pitch by 6 mm wide (tall) pulleys with the most important 5mm bore. They have 4mm and 6mm bore but not even listing 5mm bore. You must have 17tooth pulleys or else you would need to change the thingomatic.xml for the X and Y steps per mm.
