You must be joking! Surely you can source a 7812 somewhere in Russia. Actually, you must have like 10-20 shops that deliver stuff(tme.eu, ru.farnell.com etc). And they have inductors too. I understand that paying 20$ to ship a 0.30$ regulator is hard, but bulk up the order and get a lot of the stuff you need. Or you can ask the guy at the shop to order some for you. Then 3-4-5 days later you go get them.
Switch mode is great, but maybe it's easier to build a linear one, first. I mean there are a little more complicated. You try to run away from switching the windings and then you choose a switch-mode regulator that you know nothing about, lack the inductor and want to buid it yourself...
It may work and it will be awesome if it does, but I strongly doubt that you can pull it off, since you well... lack the experience.
IMHO, it's better to have a lesser perfect PSU, but a working one. I would advise beginers to build a simple 15V @1A, linear, maybe current limited PSU. When you're done, you not only have a PSU, you have experience in PCB making, fiting parts inside a box, making a front panel, making knobs not stick out too much as to show the nut holding them. Little thing like that, that will serve you very well in the future.
NOW, this is for the contest, so maybe you're better off in sticking to the hard route, give all your best and hope for success, but consider that you might bite more than you can chew.