You can buy credits with tu. So technically you can buy them with tu, but it's moderately expensive, which is where it's supposed to be. Lol.

If you want to change the name of your creatu, you have to save up the tu to get one. It's part of the challenge. And really, it's pretty easy to earn 1m a day. So in about 4 days you could have the 3.75m you need to get one Name Tag. There are guides readily available in playing the Stocks and doing dailies to get tu. Selling hatches, playing games every day, egg hunting for rare creatu.

Heck, if you go hunting for the Iluvu right now and find just one egg, you are 3/4 the way there to getting your Name Tag. lol