If you're not sure if there are any rat (or pocket pet) rescues near you, check out Petfinder. Use Find Other Animals, hit Small & Furry, and then select the "species" as rat. If nothing pops up, then make sure to expand your search from 100 miles to anywhere (if you're willing to go that far).
Animal Humane Societies typically take rats as well, but those guys are adopted out fast, or at least the one closest to me.
Even if this wasn't a rescue rat forum, I wouldn't recommend buying a rat from Petsmart or Petco. Those animals you see in stores are lucky to be alive thanks to the mistreatment and straight-up neglect and abuse that run rampant behind the scenes. Please do not support the cycle of abuse in these major chain pet stores. Just steer clear or major pet store chains that have animals for sale and not rehoming.
If you have to go to the store, first ask if they have any rats that need to be rehomed before you make a purchase. These stores will have unwanted animals in the back and not on display usually.
If all else fails, maybe check out Facebook. I don't use Facebook, so I'm not well-versed in it, but I'm sure if you search up your area and rats, something may pop up. See if anyone on there needs to rehome their rats due to allergies, living situations, or they're moving and can't take them with. I wouldn't usually recommend Craigslist, but I've seen people on there rehoming adult rats due to allergies, moving, living situations, etc. I ended up adopting a bonded male pair from Craigslist not too long ago. I don't generally go down that rabbit hole, but they were labeled free...
Of course, with all of these sort of sites, become well-versed in safety and whatnot. I usually would make Facebook or Craigslist a last resort simply because you don't know who is making those ads, but if there are no other rat rescues near you, it may not be bad to help others.
Make sure you research rats. Check out Youtube videos, such as The Rat Guru or Shadow the Rat. They provide great care and training tips. As SQ said earlier, expect to spend at least $150-$200 on the cage alone, not including the fleece/bedding, the food, toys, and hides you'll have to buy. I buy "remnant" fleece from JoAnne fabric store. I get fleece for cheaper and there's usually cute designs. This fleece will be used for hides, hammocks, cage liners, even stair liners. My rats like to use unused fleece as something to cuddle up and curl in, so fleece is very well used with my rats.
If you go the fleece cage liner route, don't be afraid to get litter boxes and actual litter (I use a recycled paper type of cat litter, so it's not harmful to their lungs and since it's cat litter, I get a bigger bag for half the price of small animal litter). Rats can be potty trained.
Good luck! Be safe if you choose the Facebook or Craigslist route. Really search and see if there are any rescues around you first.