I just ordered a new Starrett 4" double square, and while shopping around, I found a good bit of info and several sites with good deals on Starrett and other brands that are supposed to make high-end squares. I thought I'd share some of what I found here, hoping it might save someone some comparison-shopping time!
First, the reason I'm ordering one is to replace the Lee Valley 4" double square I've owned for a few months. I used it a LOT, it was a good deal, was very square, and mostly worked well, but I found that the blade slipped through the head with just a little bit of pressure, even when the screw was tightened down. LV gladly sent me a new one, which was a little better, but still didn't grip the blade consistently, so I returned both. Nothing against LV here - maybe I got 2 duds, or maybe their double squares just aren't quite up to snuff, but their customer service was top-notch as usual, and I'll certainly be ordering from them again (low-angle smoother, here I come...)
Non-Starrett brands of combination and double squares that are reported (somewhere) to be good quality, mostly USA-made: Mitutoyo, PEC, Brown and Sharpe, Fowler. These brands sometimes go for very reasonable prices on eBay - apparently a lot of people just search for starrett.
Model numbers of some 4R-graduated, 4" double squares: Starrett 13A, B&S 599-555-3, Mitutoyo 180-104, PEC 7104-404(?), Woodcraft 04P56, Lee Valley 24N08.01 (The heads on most of these EXCEPT Starrett, look like they MAY be made from the same casting, but I can't say for sure either way - anyone have more info on this?)
Sources for squares: (The first four sources below stood out for their great prices on the 4" dbl square, and after that I'll post several others with varying prices, in case one is local for you or you're ordering something else anyway to offset shipping)
BEST PRICE SOURCES: Amazon: Due to their free shipping and no tax, I found Amazon had the lowest final price on the Starrett 4" double square, at $44, but it was out of stock by the time I was ready to order. They also have reasonable prices on the Starrett 6" double square ($56) and 12" squares. There are also several inexpensive PEC squares on Amazon, sold through Victor Machinery Exchange: $25.90 for the 4" double square, $30.50 for the 6" dbl, and $43.50 for the 12" combo square, not including shipping.
Homedepot.com: Believe it or not, HD's online site had the Starrett 4" double square in stock for $44.99, and if you can find something else to order for > $4.01, you can get free shipping (free shipping over $49), but you'll still pay tax (unless, I suppose, you live on the moon where there are no HD stores).
Mansontool.com: Excellent deals on Starrett: $48.45 for the 6" combo square, $46 for 12" combo w/satin chrome blade, $41.50 for 4" dbl square, several head materials and blade finishes available.
Craftsmanstudio.com: Prices on Starrett are average, BUT - they have a 110% price match guarantee, so they honored Manson Tool's price and shipping cost, and beat it by 10% of the difference. (And they have the best prices I've seen on Lie-Nielsen planes!) This is where I ended up ordering my double square, and due to their quick communication and helpful service so far, as well as their good selection of premium hand tools, I expect I'll be shopping here again in the future.
OTHER SOURCES: Woodcraft.com: $35 for 4" dbl square, looks like LV and others, but it'd certainly be worth checking out if you have a local store and they stock this double square. Just see if the blade locks down tightly with finger pressure.
Littlemachineshop.com: Best deal I found on a 6" combination square: PEC brand, $29.50 + shipping.
japanwoodworker.com: Fairly good deals on various Starrett products, so if you live in CA's bay area or you're ordering something from them anyway, it's worth a look. I have one of their nice Japanese chisels and I love it (good thing, since it cost twice as much as my set of 4 Marples...).
grainger.com: Has Mitutoyo 4" ($42) and 6" ($45) double squares - and they probably have a branch near you.
hartvilletool.com: Average prices on Starrett 12" ($73) and 6" ($65) combination squares, but through May 31, they're including a free "Hi- Gage" attachment - if you want one of those anyway ($20 separately), they would be a good deal.
KBC tools: Variety of PEC and Starrett combination and double squares, at pretty good prices, so if you need some solid carbide end mills (read: inexpensive router bits), this would be a good source.
MSDiscount.com: Good deals on Starrett 12" combination squares - $49-73, depending on your choice of blade finish and head material.
mscdirect.com: Decent deals on 4" double squares: Starrett ($46), PEC ($32.19), B&S ($48.03), other sizes available.
nolansupply.com has PEC, Fowler, Starrett, and Brown and Sharpe squares.
travers.com, and pts-tools.com were a few other names that showed up as I searched for suppliers through the mfr's websites.
Hope this helps! I'm sure someone will point out where I could have gotten the 4" double square cheaper, but mine's already in the mail. I'm sure someone will correct me if any of this info is wrong.
Andy