Sig226 9mm and Sig226 40cals can use the same mags so is it legal to have 20rd mags in restricted states like NJ. Afterall the 9mm 20 rd mags dont say 9mm on them and if you put 40 cal rds it holds 14 rds and thus not a high cap mag.

If this is true we can buy the 17 or 20 rd sig 226 mags
where can I get the best price on quality 17 or 20 rd sig mags

I'm not sure what NJ laws are. I am sure your State has a web site and you can find the firearms laws there.

What most people note about SIG mags is a 10 round 40 S&W will hold more 9mm and work in the 226. Most agree this is legal since you don't have to modify the mag.

With Sig there are only 2 brands to rely on. Factory and Mec-Gar. Mec-Gar makes the Factory mags.

i think your trying to bend the law
but i also think most cops wont bother checking

There is no bending of any law.  Under the expired Federal AWB magazines were never restricted to specific firearms and a magazine could be modified to operate in a different firearm other then it was originally designed for.  The rule was the mag still had to function in the original firearm it was designed for.  We're talking minor mods here like the addition of a magazine catch etc.

With the expiration of the Federal AWB you can do anything you want including the manufacture of magazines of any capacity unless specifically prohibited by state law.  

As long as you stay within the 15 round NJ restriction in the pistol you intend to use it for I don't see how this would be illegal.  The only other consideration is in some jurisdictions (NYC etc.) the mag cannot extend below the grip of the gun.  I don't believe NJ has that restriction though.

You should of course consult an experienced firearms law attorney in NJ before you possibly subject yourself to a arrest and prosecution though.