where can I purchase operating system disc for Imac early 2010?

No such thing as an early 2010 iMac. 2010 iMacs were released in July and as such are Mid 2010 iMacs.

If it was purchased in early 2010, its likely a Late 2009 iMac.

Identify your iMac - Apple Support

A 2009 iMac would have come with macOS 10.6 Snow Leopard DVD.

You can buy one from Apple for $19.99

Mac OS X 10.6 Snow Leopard - Apple

A 2010 iMac would not require a system disc. It can use Internet Recovery to install a compatible version of MacOS.

It can use Internet Recovery to install a compatible version of MacOS.

Not without a firmware upgrade. They originally shipped with 10.6.3, which isn’t downloadable.

The first iMac model that supported Internet Recovery straight out of the box is the late 2011 one.

(158674)

You have some conflicting info there: my mid 2010 27" iMac came with 10.6.3 and they sell a 10.6.3 disk at the online Apple store, but it also depends on the exact build number. Your subject line mentioned "early" 2010 - there was no iMac then - the previous was a 2009 (which you might have purchased in 2010).

Best to give Apple your serial number - that way they can determine exactly which version you need.

I updated mine (and, in hindsight, shouldn't have) - it was near impossible to get it back to factory condition when I wanted to sell it because of that firmware update. I did some extensive testing and in 4 out 5 tries, I got nowhere with internet recovery and it would no longer accept an installation with the original install disk. The only thing that finally worked was my clone I had made - I was able to boot into that, wipe the internal drive, and clone the thing back.

Click here, phone Apple, and tell them that the ones in the online Apple Store aren’t compatible with your Mac.

(158673)

I was under the impression the EFI update was applied with 10.7 Lion automatically. So unless the machine was never updated to anything past 10.6.5, its very likely it has said update.