Below are some links to stores that carry announcement style envelopes that can be used for wedding invitations. Generally, envelopes come in boxes of 250.
The Paper Mill Store
One of our favorite stores in The Paper Mill Store. In their search bar you can type in the envelope size (ie. A6) and you should get a wide range envelope colors from which to choose.
The other great thing about The Paper Mill Store is that once you select the envelope you like, generally, if you scroll down to the bottom of the page, it will have the matching papers displayed.
Kelly Paper or Xpedx
Kelly Paper and Xpedx are warehouse type stores that are full of paper. It is these type of stores, that a lot of local printers purchase their envelopes. You can search for envelopes on their websites. Even better, if a store is local to you, you can go to their warehouse when they are open to the public and talk to their sales staff. Just tell them what you are looking for and they should be able to show you what they have in stock. You will actually get to look at and feel the envelope before you buy it. These type of stores also sell paper and the sales representative can also assist you with finding a paper/envelope match.
Neenah Paper
This is a popular paper manufacture that you can purchase envelopes directly from their website. You can search for paper and envelopes on their website using their search bar.
One great thing about Neenah paper is that you can order samples of paper, generally for free – although you do have to pay for the shipping. The samples are a great way to make sure you like the look and feel of the envelope before ordering an entire pack.
Your Local Printer
Your local printer may have a house stock of envelopes. Local printers are a good resource and may be able to get you your wedding invitation paper for cheaper than ordering online.
Hobby / Craft Stores
Popular hobby and crafts stores such as Hobby Lobby, Michaels, and JoAnn Fabric and Craft Stores generally carry a stock of blank paper and wedding invitation kits. The wedding invitation kits, in particular, can be great for the person who wants to make their wedding invitation themselves. In general, the kits come pre-cut with some sort of embellishment on them. They also come with the matching envelopes. Before purchasing the kit though, make sure your printer can handle the thickness of the paper and size of the invitation.