I'm actually in china and here olive oil is expensive and difficult to be found. I need it to fry slightly onion/garlic for ex. to prepare tomato sauce.
What can I use as alternative?
Nothing else is going to taste like extra-virgin olive oil, but extra-virgin isn't necessary for cooking at all. As a matter of fact, the smoke point of extra-virgin olive oil is so low that it's not often used for cooking. For cooking with olive oil, usually the choice would be "refined" or "pure" olive oil. Honestly, those don't taste like much. So whatever neutral oil you usually use (soybean, canola, peanut and vegetable are a few options) will work just fine.
My comment on the other answer got too long. Couple of points:
Regarding the "controversial" status of smoke points: I don't think it's that controversial. Chemically extracted and altered olive oils (pomace and "light" variants) behave differently to pressed, "real" olive oil. Few people include the first when talking about extra virgin; which has a sub-200°C smoke point.
You can still cook with EVOO (we do) but you can't cook like you're in China with it. It'll burn and it'll ruin your wok much quicker than almost anything else. You want to crank the temperature right down. A 600°C wok won't do at all.
If you're doing garlic to go in a sauce, cut a few bulbs in half, drench in oil and salt the bulbs. Roast them for ~40-50 minutes at 160-180°C. When it's all brown and soft, squeeze out the garlic and either combine with your oil or separate them (not sure how much oil you want).
Not only will you end up with a much richer roast garlic paste but you don't have to worry too much about burning things.
But as for the oil, if you're mixing it with garlic and tomato, I would be surprised if anybody noticed it wasn't extra virgin olive oil. Pick something that you can cook with and save the EVOO for when you would taste the oil (salad dressings, etc).
Can you access Walnut oil? I love La Tourangelle from Amazon; it adds a unique flavor and I use it almost interchangeably with EVOO. It tastes great and has a great anti-inflammatory powers similar to EVOO.
at the end I bought some rapeseed oil, this is the only thing I was able to find ( since I don't know chinese and my first concern was to find a 100% vegetable oil ... ).
It was not the best (this oil has a strong taste) choose but my pasta "aglio e olio e peperoncino" was not so bad. ( main problem was the rice pasta that has to be cooked in a different way and get overcooked ) I had to be very careful with the wok since it tends to rise high temperate and burn the garlic ( even if I set the fire to the minimum )