As Zmrzlina and Erin said, you do need the "det", and it is because in Swedish, verb always goes in the second position (except in questions).
Note that "second position" doesn't mean "second word". In the sentence "En rosa varg äter bröd", "En rosa varg" is a semantic unit.
available is a reasonable translation for finns in only one sentence in this course, Byxorna finns i rött och svart 'The pants are available in red and black'. (or 'come in red and black') It's appropriate there because in that case finns cannot be translated into an expression with 'there is/are'.
In all other cases in the course, adding 'available' would just add something that isn't there in the original sentence. I can see how this can be confusing, so I've just removed the hint that says available and added a special hint for those pants instead.
For anyone interested; 'finnes' is a deponent verb or 'middle voice', which is passive in meaning, formed by adding -es onto the stem of the verb.
So 'finnes' literally means 'finds itself' is rather 'is found'.
The ending comes from Old Norse -sk, which is from the reflexive pronoun sik (Swedish sig). Some of these Old Norse verbs found their way into English, such as 'bask', from 'baðask', literally to bathe oneself.
It has to do with the passive form of a verb, but in the case of "finna" the meaning changes somewhat in passive
1 active to buy = att köpa
presens: köper, preterite: köpte, supinum: köpt
passive to be bought = att köpas (note just add -s)
presens: köps, preterite: köptes, supinum: köpt
2active to find = att finna
presens: finner, preterite: fann, supinum: funnit
passive to be found to be (situated) = att finnas
presens: finns, preterite: fanns, supinum: funnits
There are a few different types of verbs which end in s. In this case it is, like kirakrakra says, a passive form. In English you need more words to say the same thing. An example: "Djungelns lag: Att äta eller ätas" "The law of the jungle: To eat or to be eaten" So "det finns" can be translated as "it is to be found"