My 2 months old baby does not get enough breast milk. He loses weight consecutively. Can I give her alternative milk from market?
First and foremost, a 2 month old should not be losing weight, speak to a healthcare professional (midwife, doctor or both).
To answer your question, properly formulated stage 1 formula milk is suitable from birth, but it's best to discuss this with your doctor or midwife as there might be other things you want to try.
And something else to think about, have your child checked for tongue tie. This is a condition where the tongue is tethered to the bottom of the mouth (more so than it should be). It is quite common and is frequently missed. This condition causes difficulty with breast feeding and is very simple to fix once spotted. Even if you decide not to continue with breast feeding, it's worth getting this checked as severe cases can cause issues with speech and it's much harder to fix later on.
EDIT: I don't have enough rep to comment, so I'm adding this to my own answer: aparente001 makes some good points, I didn't know about the supplementer whatsits, that sounds like a good idea if you are hoping to continue breast feeding. What I really wanted to add was that Lactation consultants are also a thing in the UK, but you have to ask your health visitor or midwife for a referral. They tend not to be offered unless you ask. Since we don't know what country the OP is in, this may be useful information.
If you want to supplement breast milk with some other milk, make sure you buy baby formula, either based on cow's milk or on soy milk. Don't use plain cow's milk.
Express milk from your breasts often, so you can increase how much milk your breasts are making. You can even give the expressed milk to your baby as a supplement!
The best way to give a supplement is with a special gadget that delivers milk to baby through a thin, flexible tube, which you line up with your nipple so that baby takes the nipple and the tube in its mouth at the same time. The reason the tube needs to be very small, is so that the milk only flows when the baby is actively sucking. (If the milk flows too easily, the baby will become lazy, and not suck hard enough on the nipple, and then your breasts won't be stimulated to make a lot of milk.)
There are two brands of supplementer gadgets: Lactaid: http://www.lact-aid.com/ and SNS: http://www.medelabreastfeedingus.com/products/51/supplemental-nursing-system-sns
In addition, you can use a large syringe (with no needle, of course), and push the plunger in very slowly, as you are nursing. This is something you could start doing very quickly, because you can buy the syringe in your pharmacy right away. The supplementers usually have to be ordered on the internet.
Finally, in an emergency, you can use a sippy cup: and pour the formula very slowly into baby's mouth. If you don't have a sippy cup, you can use a paper cup and pinch the top to make a little spout.
If you feel you must give the baby a bottle, please try to find the kind of bottle that doesn't encourage lazy sucking. I'm sorry, I don't know a brand to suggest. What you're looking for is a bottle that makes the baby work hard at sucking -- without frustrating baby TOO much.
In the U.S. you can work with a midwife or lactation consultant to help you figure all these things out. If you are in another country, could you ask if there is someone who gives this kind of assistance?
If you can find a chapter of La Leche League where you live, that would be great. They can help you.