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basically if you are short on materials you can adapt a 3mm bolt. you can chop the head off the bolt and use it as a shaft.
you can also go to the home store and buy some nails that measure out to about 3mm(just under) so you can cut the head and pointy end off and use it as a shaft. you can buy ready made 3mm rods and chop them to size as well. it is also possible to 3d print the shafts.
all these options give you flexibility to help you create the truck however depending on the method for the shafts you have pluses and minuses.

the best option is to purchase some 1mm, 2mm and 3mm shafts they come in handy for different projects. for the ursa bear you will need 2mm and 3mm shafts to create the strongest and tightest drivetrain. 3mm is used on the main shafts and 2mm is used for the small bevel gears and if you decide to convert to CV driveshafts. everything is optional so it will be up to you how far you want to take it.

loose tolerances in the gearbox can cause the gears to skip or become damaged from incorrect meshing under load.

additionally using Nylon in some key areas helps to increase the trucks durability.
PLA is ok but doesn't take shock impacts very well (it cracks)

changing the small bevel gears, steering servo arm parts and the wheel axles to Taulman 910 nylon will remove some weak spots that PLA or PETG can cause.combined with proper 3mm and 2mm shafts and converting to 3d printed CV joints can actually bomb proof the driveline , free up friction and make the truck drive much nicer.

the truck is a really good design that allows for a lot of flexibility Tahustvedt really came up with a winner here. so take it slow take your time do it right and you will be amazed.