It really depends on what sort of hens you want, ie purebred or hybrid layers, and when you want them to start to lay. Purebreds are generally hatched in the Spring and thus are coming up to lay by about August - September. Because of the time they are hatched, young purebreds at POL are usually not available until after midsummer. You can buy them as growers, ie 10 -16 weeks old, and bring them on until about 22+ weeks when they start to lay. At that age they will not be so expensive as POL pullets. You may be able to buy some now, which haven't started laying yet, but these will be unlikely to lay before the days begin to lengthen after the solstice. You'll have to grow them on and feed them all through the winter, which is OK if you want to do this and gives them time to settle in.
Hybrids are often hatched in Spring, just the same as purebreds, but some breeders hatch in November, overwinter the chicks/growers under cover, and have them for sale as POL by Feb.- March. So POL hybrids are available nearly all the year round. However, many people prefer birds that have been hatched in Spring because they've had the advantage of living outdoors in sunlight and eating Spring grass (if not raised in intensive commercial conditions.)
As Tygresek says, there are quite a few posts about introducing chickens, but basically it boils down to this;
1) Divide your run or put them in a separate run + coop if you have one, for at least 2 weeks, to see if they have any health problems they might pass on to your birds. If they can see the older birds, this may help when you integrate them. You can make a temporary box for them to roost in, no perch needed, so long as it's dry and safe for them at night.
2) Don't put them in with the older hens if they're much younger as they will be bullied. If the others are adults, keep the youngsters apart until they are at least 20 weeks and can stand up for themselves.
3) Put them in the coop with the older birds after dark, when they are all asleep. Sleeping together and waking up as a flock will help them to be accepted, whreas if you just bung them in to the run in daylight they'll be attacked as intruders. Watch out for trouble, there will be some chasing and pecking but this is normal. Only intervene if things get nasty, ie blood is shed. Provide extra feeders and drinkers at first, and if the newbies look a bit frazzled or you think they're not eating properly, give them an hour as 'time out' in their original enclosure to rest and feed.