My experience has been that it near impossible to buy affordable quality arbors that run true.
Straight shank arbors are quite easy to make, and for most hobby applications you can ignore the keyway. Then you can grab them in any collet or even an end mill holder.
I've made straight shank arbors for slitting saws, involute gear cutters, grinding wheels, etc.
There are a few key thing I learned making them:
First is to bore a hole in the arbor the same diameter as the blade that will be held, and make the "retaining cap" a plug that fits very closely in the bore.
Under cut the holding face of the cap, and possibly even the arbor so that it clamps only on the outside diameter of the arbor face. This is important for a few reasons - without it you can break a slitting saw (DAMHIK), and it helps with transfer of torque to the cutter by moving the clamping forces out as far as possible.
True up the body OD in the same operation as making the bore to have as concentric an arbor as possible.
Once I learned to do those things my arbors have been great to me. Some have been made where I threaded the plug itself and gave it pin drive holes (and made my own wrench), and others are just simple through holes to allow a standard cap screw. Depends if you need the clearance on the bottom, and how rushed you are to get the job done!
Attached are a few pics of arbors I've made. I tried to standardize on a 1" diameter shank, but you can use whatever suits you.