Learn Dutch: Ik zoek een wasbare deken voor de hond. - I am looking for a washable blanket for the dog.

It is built from the stem of the verb wassen (to wash), which is was. To this stem, we add the suffix -baar, which expresses potentiality (just like "-able" in English), creating the adjective wasbaar (washable).

Why is it written as wasbare with a single 'a' rather than wasbaare?

Because deken is a de-word, the adjective in front of it gets an -e ending, even after the word een. When you add -e to wasbaar, the syllables break as was-ba-re. Dutch spelling rules dictate that you drop the double vowel in an open syllable to keep the vowel sound long, so aa becomes a.

The English translation says "I am looking for", but the Dutch sentence just says Ik zoek. Don't I need the preposition naar?

You can use zoeken naar, but in Dutch, zoeken is very frequently used as a direct transitive verb without a preposition. Both Ik zoek een deken and Ik zoek naar een deken are perfectly correct, but omitting naar is very common and natural in everyday speech.

Could I also use the present continuous here, like Ik ben een wasbare deken aan het zoeken?