First of all, think over if you would like a "purely" pet mouse, or possibly a mouse to attend shows with. As a pet and in pet classes it does not make any difference if the mouse is of Pet (American, Swedish.. the name varies)- or English type. In standard classes the mouse should be as close to the standard both when it comes to the coat, color, markings, type and size. Usually the required type is English
The best way of obtaining a standard class show mouse is from a reputable mouse breeder. Mouse clubs usually list breeders and you can contact them for reference. With breeders you will get information on the mouse's background and family, many will provide you with a pedigree. In some countries a mouse has to be registered in order for you to show it and breeders will have the mice ready and registered. Good breeders are also able to help you with all kinds of questions regarding mouse keeping
Many breeders have excellent and fun looking pet mice as well, as "pet mice" (as opposed to mice who can take part in standard classes!) are always been born in litters. Some breeders also will let you have a joint ownership or similar to offer. Sometimes even very good show mice, usually bucks, are rehomed for responsible mouse owners for free.
When you get a mouse from a breeder, you will know the exact time of birth and you are able to avoid many unpleasant surprises which may happen with a pet shop. A breeder will tell you the mouse's sex for you, she will not sell you a male - female couple instead of two females, or a female that is already pregnant. To put it short: it pays to use the service of a breeder!
Last but not least: there are all kinds of breeders around. Some are responsible and have a good reputation. Others could be better classed as mouse millers. Ask around for references, especially clubs. If possible, go and visit the breeder and keep your eyes open!
Mice are ready for new homes at one month of age. You should not sell nor buy mice younger than this. Mice who are old enough to be sold already have the looks of a young adult. They don't look like babies with large heads and skinny legs. You should be able to determine the sex quite easily; buck will have their testicles showing, females do not. Females have a short distance between the anus and the urethra and you may be able to spot two rows of teats on both sides of the belly. Young males can lift their testicles back up so you can't see them, if they get scared, but you will still be able to determine a male from a female by the longer distance between the anus and the urethra.
When in a shop, it is best to ask when the babies were born. They should know this fact, at least roughly. Do not buy too young a mouse! If there were males and females housed in the same tank, be prepared for babies if you decide to get females. Even though it is possible that a female mouse gets pregnant at one month of age, she is not ready for pregnancy until much older, 3-4 months old.