These are fairly hard to find - they're published by World International, the same publishers of the annuals and "The Fruit of Evil" and "One Enchanters' Eve" books (but are called Mini World because they're smaller - the size of the mini comics).
They appear from time-to-time on eBay. I wouldn't say "Catra's Prize Catch" is the most common. It was the last one I lay my hands on - I always felt "Princess Adora Imprisoned" was the easiest to find. The books are good and are some of the few publications that feature Scorpia (she's in two of them, but only drawn in one - "Threat to Crystal Castle" is especially good).
The other book is "The Red Stone of Darkmor".