Helvetica is a sans serif font that is virtually identical to Arial with maybe a very slight difference in relative size.
It may be that the website people use Macs because Helvetica is provided with Mac OS but not Windows XP click here. Any font that can't be relied upon to be on every PC or Mac is a bad choice for websites. click here for preferred website fonts.
I can't find an open source supplier but if you really must have it (despite the problems mentioned above) then email me.