Should really go with a spare of same type, size and load rating otherwise you really just have a temp tire to let you limp your way to a tire store...  Which could mean a wait while they order you one.  My two cents anyway.  My NTU Quicksilver came with a bias spare of a different size.  I dumped it and got a spare to match when I replaced the tires.

Etrailer is a good resource for parts like this.  You can order them already mounted to rims, which can help if your existing rims are old, rusty or scraped up.  For your size, it looks like they only have them mounted to aluminum rims, though -- not a problem, just a little more $$.  Check the bolt pattern carefully before ordering.
http://www.etrailer.com/Tires-.....31206.html

You can also order just the tires and have them mounted locally.  Most local tire shops should also be able to order them for you.  Most of the ones around me can typically receive them the next day, or two. 

Aside from that, I agree with everyone else, stick with the radials.  You're right on with the weight issue, too.  The guys at Walmart clearly don't know what they're talking about.