10 lbs is not safe weight loss for your body, and I would say that it's only unattainable for people who have a lot of body weight- and it isn't safe for them either. Please, be safe, healthy, and focus on cutting out bad foods (sugar) as that's most of the battle! You can always get your dress adjusted.
Theres no point in losing this weight, like this. You can lose 10lbs over 2-3 months, and have a better chance at keeping it off. I'd dress shop as you are, start eating healthy and making some changes, and once you are ready for your alterations they can fit the dress to you instead of the other way around.
I did the keto diet and I actually lost 21 pounds in 3 and 1/2 weeks. Keep in my I had carbs and sugar for every meal so I trimmed down right away especially in the stomach area. It is suppose to be one of the nest diest but it is a lifestyle more than a crash diet. Good luck!
On my body, 10 pounds wouldn't make a big difference as far as fit and style of dress, so to me it seems unnecessary to try to crash diet for a shopping trip. It's also likely that any 10 pounds you lose that quickly will be mostly water and come right back once you loosen up on the diet. If you have just 10 pounds to lose, I would focus on habits and stepping up your workouts.
This is not going to happen safely unless you are 50-100lbs overweight. As a PP said, whether you lose 5-10 lbs is not going to make a difference in the size that they order for you. What will make a difference is how much you lose by your first fitting. I would focus on healthy weight loss, 1-2 lbs per week.
If you do it safely and space it out , it can be done. Cut back on sugary drinks and unhealthy snacks. Drink more water. Eat healthy snacks if you need something sweet. Move more! Go for a walk, take the stairs when you can. Also I seriously recommend you download an app called lose it! It is seriously awesome. Helps you track your weight loss, food and exercise as well. Also MapMyRun is great for tracking just your workouts.
I've been on a raw vegan cleanse (no carbs, meat, dairy, artificial sugar) for 15 days now. I would recommend it. A cleanse group member lost 2 kilos during this time purely off diet alone (no exercise).