Try Tiffany and Co. They use a 20lb weight. They are not inexpensive but it is well worth the price. They will guide you as to which size to use. There are also American Stationary but they have nice items but not up to Tiffanys.
I agree that Tiffany make beautiful personalised stationery. I think Mr Pipps is looking for a more affordable option, which excludes engraving. Tiffany will not do letterpress. They have a broad selection of stock weights, so you can have lighter or heavier paper based on your preference.
An important, but perhaps trivial, distinction to be made between premier American stationery maker and their British / European counterpart is what the respective group think is the ideal rag content. Americans tend to think that it should be 100% rag. Over here, people would argue that it is better to have 50 to 70% (depending on whether you listen to the Brits, French or Italian), pulp content. I do not take sides on the issue since I think it's one of those things that you simply agree to disagree.
Another consideration is how you prefer your envelope. Over here, it is popular to line envelopes for social stationery with coloured tissue. Americans do not have this habit. Again, I think that this is a matter of preference.
How should you decide? I would consider who the majority of your audience is.
Going back to Tiffany, while they do make beautiful stationery, I do not believe that they represent good value. Let me explain. They used to make all their paper in-house, in New Jersey. (They even made their blue boxes themselves.) Somewhere along the line, this became economically undesirable. The paper is now supplied by Crane. Instead of the Crane watermark, it has the Tiffany & Co. watermark. They also used to do not only the engraving in-house but also cut the engraving dyes themselves. Now, the dye-making and engraving are done by a specialist called Excelsior, whom Crane acquired some years ago. In other words, everything is done by Crane and their subsidiary. Therefore, if you go to Crane, you should get the exact same quality for less money.
I say "should" because it is possible that Crane might do the engraving a certain way. Many new consumers of engraved stationery insist that there is obvious bruising on the back of the engraved area so that it is obvious to even the untrained eye that it is indeed an engraved stationery. Crane may cater to this preference by default -- I do not know this for a fact but merely mentioning it as a possibility. Traditionally, highly skilled engravers endeavoured to minimise the bruising and as such, obvious bruises were considered to be a mark of poor workmanship. Tiffany understand this so they will work towards the traditional ideal unless the client asks for look-I-have-engraved-stationery bruising.
Apologies for the rambling post...
