An obvious place to start is in a bridal boutique.

Bridal Shops/Boutiques First of all, sit down with your fianc and/or your parents and anyone else that will contribute financially to your day and decide how much you want to budget for your wedding attire. For some idea of the prices you could pay in the UK, an off-the-peg gown from a UK high street shop could, at the time of writing, set you back anything up to around 400. A middle-of-the-range designer wedding dress could cost between 300 and 1500, and haute couture and other designer dresses can cost thousands of pounds. One important thing to bear in mind is that if you don't tell people, nobody will know what you spent on your dress - so budget for what you can afford. You will certainly be able to find something suitable in every price range, and it is best to set off with a budget in mind so you don't accidentally try on and fall in love with a designer number that completely breaks the bank.
There are numerous options when it comes to buying a wedding dress, and which you go for will depend on your budget, taste, and the amount of time you have before the big day.
An obvious place to start is in a bridal boutique. Many of our towns and cities have these kinds of shops somewhere on the high street and in your pre-engagement days you may well have walked past one many times without noticing it. There are several large chains of bridal boutiques that sell their own range of dresses, and there are smaller, independent shops that will sell gowns from a number of different designers. If possible, try to go to places that have been recommended and ask around to find out if anyone you know has used a particular shop before. Many shops will have a website, and it is a good idea to have a look at that before you go as it should give you a list of the different designers stocked by the store this will also give you some idea of the prices of the dresses you are likely to find there.
Increasingly, high street stores are producing good quality bridal outfits off the peg. These are usually considerably cheaper than the dresses in bridal boutiques, but are often no less beautiful. An advantage here is that the stores often stock the dress in all sizes so you can try on the correct size for you before buying. Also, there is no waiting period for the dress to be made, unless you need to order a size that has run out, so these dresses can be bought much closer to your actual wedding date. This is handy if you are having a short engagement or are planning to change size in the months leading up to the big day. It is important to check the dress that you buy for any damage it might have sustained while being tried on in the shop.
There are many online stores for buying wedding dresses, although not every designer allows their dresses to be sold in this way. A popular method of online shopping from the UK is to try on the dresses in the shops then order online from the USA. Prices are usually lower from the American stores, even when import tax and VAT are taken into account. If you are feeling brave then you might like to bid for a dress on one of the online auction sites and there are also sites where it is possible to buy once-worn dresses. This is a great way to find a designer dress at a bargain price but pay attention to the height of the bride who is selling the dress! If she is several inches shorter than you it is unlikely there will be enough material in the hem to adjust it to be the right length for you.
If you have visited all of your local bridal stores and have been unable to find what you are looking for you may want to consider having a custom gown made by a dressmaker, a talented relative or yourself8. It pays to get recommendations if you plan to use a dressmaker, and if you aren't able to use somebody that you already know then make sure that you obtain and check up on their references. Most dressmakers will be happy to show you samples of their work and should be able to put you in contact with brides they have worked with in the past. The costs for having your dress made can vary dramatically depending on the type of dress you want, the kind of detail that you are after and the material that it is made from but compared to some designer dresses this can turn out to be a very cost-effective option. The major benefit of this method is that you can have exactly what you want and you know it will be totally unique.
Several of the major charity chains in the UK have a shop devoted to bridalwear, and it is well worth hunting these out as they are the perfect place to find a bargain. You might be lucky and find the ideal dress, or you may find one that can be altered by a dressmaker to give you something beautiful and unique. Charity shops are also great places to pick up vintage dresses if that is more your style.