Thanks for recommendation.
The nearest electronics shop. eBay, Amazon will overload You with sellers and items....
What part of the world do you live in?
Assuming you are in the U.S...
...I usually check Jameco first, just because I've been buying from them for about 40 years!
These are all reliable suppliers. They have datasheets and they don't sell surplus stuff, "cheap junk", or counterfeit parts, etc.
The first 3 are mostly hobbyist suppliers. SparkFun and Adafruit both have lots of example-projects and Adafruit has lots of educational information.
Mouser and Digi-Key are major distributors and when it comes to electronic components they have "everything" ...If an IC is in current-production, they probably have it. And, they do sell in small quantities to hobbyists.
You can find lots of cheap junk on eBay, often without complete documentation. Amazon is the same, since most small electronics parts (and that kind of thing) is from 3rd party sellers. Of course, Amazon & eBay sell good stuff too but you have to be careful.
It's OK to buy cheap junk if you just want to experiment, or if you know what you're doing and you just want to save money. But, it can be frustrating if you are trying to accomplish something, or learn something, and you don't have complete documentation and maybe you don't have complete part numbers so nobody here can help you find the information, or maybe the documentation doesn't exist, etc.
Surplus dealers will tend to have mid-sized lots of out-of-production parts and modules; a relay where the exact version is no longer sold. Some of them also import/distribute foreign components like Arduino-compatible modules, perhaps with a somewhat better shipping time and return policy than the actual overseas vendors. Sometimes you get real bargains. Sometimes you get a wonderful part and realize that they've sold out and you can probably never get another one. Sometimes you get a part that has poor or non-existent documentation, that no-one has figured out how to use.
They're a swell way to build up your supply of random-ish parts with things that you might never use. (and they're going out of business (at least in their brick&mortar forms) at a furious rate, as people just buy things direct from China instead... Sigh.)
westfw: Sometimes you get a wonderful part and realize that they've sold out and you can probably never get another one.
OP has to realise that for his project. He needs to by the spares as well for his access control system as he's more or less selling his project (possibly at cost, no idea); if something breaks in a few years time, he needs to have the parts to fix it.
Relays are one of those parts that you may think are awfully “generic”, only to discover that the version you bought has some sort of weird pinout that everyone stopped making, back in the 80s…
Or you design a nice housing for your project and when you later buy replacements you find out that sizes of boards or locations of mounting points have changed :)