126mm is the width of a 6 or 7-speed wheel. There are plenty for sale on Craigslist priced from about $15 and up.

Craigslist is a good place to start. Prices are usually lower than eBay, and you can find a local Seller so you don't pay shipping charges. You also get to actually see the wheel/wheels that you will get, and don't run the risk of buying a piece of junk.

2nd choice would eBay look for a Seller local to you who you can pick the wheel up directly form instead of shipping. The disadvantage here is that you don't get to see it before buying. The risk being that you buy a piece of junk, and the shipping charges (that you eat both ways are more than the wheel is worth).

3rd choice would be eBay, looking for a Seller who offers free shipping, although the auction prices are often higher than auctions where the Seller does not offer free shipping, you won't have to worry about going to pick up or the cost of shipping charges (which can be very high because of the size of box used to ship wheels). Again the danger is that the wheels arrive in less than spectacular condition and you end up eating them or sending them back (on your own dime).

I've bought several sets of wheels off eBay in the past year for less than $50 a set, and they were all good transactions. The set of wheels I paid the most for were a eBay transaction. The wheels were in bad condition when I picked them up (Seller met me 1/2 half, as he was driving this way anyway). The cassette was worn-out and the cones and ball bearings were all pitted.

My best transaction was a CL deal. I got two sets of new Nashbar 700C 8,9,10 speed wheels still in the boxes (that's pairs of wheels), plus some tubes and brake pads for a total of $20. That was the asking price in the CL ad and the Seller was located less than 20 miles away from my house. +I didn't even bother to try to bargain the Seller down after I saw what was included. Just handed him the money, said "Thank you", and loaded my car up.