Two parts manual links below showing the different bolt types. The NAS series is usually more available as the industry moves away from the AN standard to the NAS standard. Price is still dependent on source/quantity but Aircraft Spruce usually keeps both types in stock. Some aviation parts houses are selling bolt kits for for the AD that include bolts, washers, and nuts.
Thank you....yes they sure make it as confusing as can be!
I was told there were several kits listed on one of the piper type forums. But your mechanic should be able to procure the proper hardware with no issues.
Aviation hardware is manufactured per various specifications or standards like AN (Army-Navy specification), MS (military standard), NAS (National Aerospace Standards), etc. Each standard or specification has certain levels of material strength and tolerances which usually start at a lower level then upgrade up to higher levels. In the case of hardware, standard AN hardware is at the lower end of the strength/tolerance spectrum and NAS or MS is at the upper end of that spectrum. Provided you always upgrade the hardware to the higher level you can substitute a lower type bolt with a higher type bolt. So for your spar bolts, while the AN176 is original, that bolt can be legally substituted with an NAS6606 bolt since it is a stronger bolt.