Your best bet in adding another pup to the family is (1) Don't get the puppy from the paper or a back yard breeder or an irresponsible breeder who doesn't health test and (2) To wait until the puppy you have is about 16 months old so that you are able to give the one you already have all of your time and attention in these critical first months that it needs. This will be especially important when your puppy hits the 6-8 month mark through 12-16 month mark which is considered their "terribe twos or teenage months" and can be difficult. At 2 1/2 months old her trainig for life hasn't really begun yet.
If you really just can't wait until the one you have is 16 months old then please don't buy one from the paper, or the pet store or from someone selling cute puppies with a sign by the side of the road, that is (1) supporting the poor treatment of a good portion of the animals involved and poor breeding practices by back yard breeder who are destroying this breed and its characteristics and (2) stand a MUCH better chance of getting a sickly dog now or in the future as health testing on parents was not done (most of thse dont show up until between 1 and 2 years of age most of the time).
Good breeders who are trying to better the boxer breed and reputable breeders who show and/or health test their boxers before breeding never sell their puppes in any of the above ways. You will read many horror stories of BYB dogs on here and its heartbreaking. You potentially set yourself up for a lot more vet bills and a lot of heartache in a boxer that died young when buying from those questionable srouces. Yes those problems CAN happen in well bred health tested animals but MUCH less oftern and if/when they do a breeder can help you and will want to help you.
This is all my opinion but i honestly feel is your best option.
oh woops, a revers brindle is a brindle who has a lot more black and therefore looks like a black dog with often a small amount of fawn stripes, they are much darker in color and sometimes can even look black at their darkest. (black is not a geneticlaly possibel trait in boxers but they can sometimes look it in a very very dark reverse) a normal brindle looks like a fawn boxer with black stripes.