You need to read the patch notes before the patch is released (they're usually online a few weeks before) and then grind the tanks that get replaced/moved. For example if you had the KV with the KV-2 turret researched, the KV-3 and the IS-4 in your garage when the latest patch (7.3, released in May) was released you would get 3 free garage places as well as 3 new tanks with a 100% crew (the new tier 5 KV-2, tier 6 T-150 and tier 9 IS-8, worth more than 5 million credits in total). So reading patch notes is not the worst thing you can do. Personally I only got two garage places as I couldn't get to the IS-4 that fast. But now it's too late of course, the patch is out already. I don't think 7.4 will have anything like that unfortunately, but I read they want to overhaul the TD lines of German and Russian tanks to split them in two in the future - it may be a good idea to research the existing tank destroyers now. Bonus codes are rare and you have to check the web regularly to get one (google). So I have no link, sorry. Like I said, IF there is one available it is usually limited to a few codes and you have to be fast to get one. Finally you can of course spend a bit of money (paysafe card is possible if you have no credit card), for example for 7 Euros you get 1500 gold, which is enough for 10 garage places if you wait for a special offer. And 15 garage places are more than enough for more or less everything since most tanks are mediocre at best and you sell them anyway once you researched the next one.