Electrical – Mysterious Wires

The maximum current for 1 AWG used in power transmission is 211 amps.(source) 70% of that is 147.7 amps.

2 AWG is rated at 94 amps, and 3 AWG is rated at 75 amps.

So there is no smaller wire rated at 70% or larger. You'll have to use 1 AWG (or larger) for all wires.

You could try an electrical supply house for other colors of 1 AWG, but I've found they only sell big spools (250+ feet) of such wire.

Try contacting a commercial electrician. They often carry those big spools around and might be willing to sell you a few feet of it. Of course they'll also want to charge you a $75 delivery fee and a $95 installation fee, but you should be able to avoid the first by going to them to pick it up, and the latter by firmly insisting that you don't want them to do it. Standard counter is they'll tell you it is illegal/dangerous/immoral for you to do it yourself. Stand your ground.

Since you have a 2" PVC (assuming schedule 80) conduit here, providing 742 mm2 of room for wire fill, and your 3 2AWG wires (assuming THHN/THWN or XHHW here) only take up 66*3 = 198 mm2 of that space, you have plenty of room to stuff a bare wire down the conduit to bond the subpanel and main panel grounding bars together. A 6AWG bare copper wire will do the job, and should be fishable through the conduit with the power to the main panel turned off. Connect it to the ground bar in the main and sub panels, turn the main panel back on, and you're in business. (Of course, if you don't want to play "fishtape wrestler" for a day, you can pay the nearest friendly electrician for that service instead.)

P.S. no, you do not need to remove the ground rod once you install the bonding wire between the two buildings -- in fact, current Code requires outbuildings to have their own earth ground, in addition to the bond back to the main panel's ground. (This is done to keep all the wires to the outbuilding from soaring to some crazy-high voltage in case lightning strikes near the circuit conductors.)