Bei is right in this sentence. Fiori is the plural form of fiore, "flower", therefore the preceding adjective bello (bollo is a kind of stamp, such as the stamp that you put on an envelope or an official document) is modified to match this form. In this case, bei.
Now, for plural masculine nouns you can also have begli and belli.
You use begli when the noun begins with a vowel, with an s-, a z-, an x- or with a few combinations of consonants:
You use belli when the adjective is placed after the noun.
Begli stivali > stivali belli
Begli zigomi > zigomi belli
In all the other possible cases, you use bei. Nouns that begin with b-, c-, d-, f- and all the consonants that were not mentioned before belong to this group.
And so on.
For feminine nouns the choice is easier, because there's only one plural form: belle.
Belle scarpe, belle sciarpe, belle tende, belle tavole... scarpe belle, sciarpe belle...
Bel, however, is one of the two possible singular forms, the other one being bello (sometimes contracted to bell').
You can say Il più bel fiore, the most beautiful flower, but you have to say I più bei fiori when talking about more than one flower. Notice how the article behaves the same way, taking a plural form to match the plural noun.
Hope this helps!