Another big fan here of Paul Stuart. I've found that with PS, you get what you pay for: the higher priced items are really good quality and generally worth the extra money.
Belts: They sell $50 calfskin belts that are only "OK" in my opinion; on the other hand they have $150 calfskin belts that are fantastic. Today, in fact, I'm wearing a lambskin belt that I bought at PS when I was in college in the spring of 1988. It was about $75 back then - expensive, yet the belt is still in perfect condition. It wouldn't surprise me that the cheaper ones were made by Trafalgar.
Sweaters: I've always thought that the "made in Italy" ones were made by Avon Celli whereas the "made in England" ones were John Smedley. Great stuff.
Shirts: I too had heard that the made in USA shirts were by Gitman Brothers, but that was a few years ago. That's not a bad thing -- PS supplies the fabric and dictates sizing and specifications. (Gitman makes nice shirts -- I have a fantastic non-PS Gitman-labeled shirt, with MOP buttons. Unfortunately, Gitman also makes lower-end shirts that tarnish its higher-end.) The made in USA shirts only have plastic buttons, but at least they are sewn on well and I can't recall ever having buttons break at the cleaners. The PS "Cooper-collar" shirts (made in USA) are some of my favorites. Now, the Made in Italy shirts are a different story -- they cost at least twice as much ($250 or so). I thought that perhaps they were made by Lorenzini, but I have no idea. Great fabric and nice MOP buttons. Keep an eye out for made in Italy "Stuart's Choice" shirts on Ebay.
Suits/Sportcoats: Made in Canada ones are Samuelsohn; Made in Italy ones are Isaia. All well made, but Isaia ones are fantastic (if expensive). When you buy a made in Canada PS suit, don't think "I'm buying a Samuelsohn suit" -- you're not. Samuelsohn only does the cut and sew for PS. PS supplies the fabric and the specifications -- Samuelsohn just follows directions, cutting the fabric and sewing together the suits and sportcoats. I assume PS has the same arrangement with Isaia.
Slacks: I don't know who makes them. They mostly seem to be made in Canada these days. I had assumed they were also made by Samuelsohn; the above reference to Barry Bricken is a first for me. In my experience (I have perhaps 20 pairs of PS slacks), the fabrics are fantastic. But, as noted, you get what you pay for with PS -- the cashmere blend super 150s slacks for $350 will be sublime, as they should be. The $75 PS khakis will be very nice, but perhaps not as nice as $125 Incotex khakis. My experience with PS slacks is that they tend to have a fairly generous fit, comparable to Kiton or Brooks Brothers. By the way, I also have some PS shorts that are fantastic - buy-em.
Socks/Underwear: The socks are gorgeous, comfortable and unique. Unfortunately, my experience has been that the OTC socks aren't quite long enough for my long legs, so they slide down over the course of a day. I have thought that at least some PS socks were made by Pantherella based on the similarity of designs. Cotton fabric used in PS boxers and briefs is super-soft. Recommended!
Shoes: Dress shoes are mostly made by Grenson. Superb shoes. There are other makers though for crocodile skin shoes and casual shoes. A couple years ago someone sold off a large batch of PS driving shoes that the seller swore were made by Alden -- but, who knows.
Other stuff: Various items that any dandy will appreciate: handpainted cufflinks, alpaca hats, jackets/slacks, etc. in wonderfully unique and colorful plaids, cashmere socks, dress gloves that are crocodile (top) and calfskin (bottom) with vicuna lining (@$650), mink-lined casual coats, fantastically ornate silk scarves, and an amazing array of belts in a multitude of colors and skins.
Finally, IMO, very attentive customer service.
... For Christmas, I just tell my wife: "Get me anything from Paul Stuart." It's hard to go wrong.