It's not easy but, sometimes, the Mistery Packs drop an Ultra part, here is mine, i got it before the last patch (Mistery Pack were sold for 50.000 IGC).
If i can i'd like to suggest you to do NOT buy more than 3÷5 mistery pack every time you log into the game.
The same for Platinum, Gold, Skill and Aftermarket Packs.
The reason is, when we could buy boosts for few €uros (Poland boosts) many of us were used to buy a lot of Gold Packs looking for good performance parts, i noticed that often after buying 3÷5 card packs i got a good part, well, continuing to buy packs was a waste of money for at least 10÷20 card packs, before i could get another good part.
The same is for Mistery Packs, so my suggestion is buy few packs each day.