There's been some discussion about it in several other threads. Try here for example:
http://www.ultimatemetal.com/forum/testament/422867-how-many-units-has-formation-damnation-sold.html,
http://www.ultimatemetal.com/forum/testament/334366-has-nuclear-blast-ever-had-band-sell-gold-record.html,
http://www.ultimatemetal.com/forum/testament/822461-droe-sales-charts.html. There is not much reliable sources listed there, though.
As for Practice and Ritual, they've both been rumored to have surpassed the 500.000 mark for a long time (Souls of Black would be the 3rd best selling record). Still, in order to get a certification after so many years from the release the record company would have to apply for it to RIAA. The band has reasons to believe that Atlantic Records is not interested in doing that.
Here's an excerpt from an interview with Eric from 1999 (
http://www.metalupdate.com/features.html)
Metal Update: 'Practice' was the biggest record, right?
EP: Yeah. So far.
MU: (laughs) Did it go gold?
EP: No, it hasn't gone gold yet. It's a weird thing, because that album came out before SoundScan. We were up to about, almost 300,000 before SoundScan, and now SoundScan is showing the record at 100,000 and something. Combine those two and its like, gold. A couple of them are. But who knows what Atlantic Records is doing. We're not really breathing down their necks. If there's a dollar lying on the table and no one's claiming it, they're not exactly gonna go "hey, man!"
And here's Chuck's interview with Metal Exiles from 2007 (
http://www.blabbermouth.net/news.aspx?mode=Article&newsitemID=75647):
Metal Exiles: What was the highest that you sold while you were signed to Atlantic?
Chuck: It is hard to say because SoundScan was not around for the first three records.
Metal Exiles: I know it was out for "The Ritual".
Chuck: It was out for "The Ritual" and I think it was out for "Souls of Black". "The Ritual" was 485,000 in the States then and who knows, it could be gold by now but Atlantic is not rushing up to us with gold records. I am sure it is still selling because kids are buying it at in stores and I see it still on the shelves. I can picture Atlantic coming up to us and telling us that they have had these gold records for awhile sitting in closets. "You want them?"
I'm pretty sure there are more interviews where they talk about album sales, I could only remember these two off the top of my head.
Generally, the best solution would be to register with Nielsen Soundscan and get the precise info on the numbers. I don't know how much this costs for someone who's not a music industry insider. If you do register, please feel free to share the info with us, 'cause there are a lot of us here interested in getting to know the exact and current figures
.
Some data from NS was being published on Metal Sludge as Sludge Scans. Those haven't been updated in years, unfortunately (I've sent them emails with questions concerning the sales of Testament albums, but received no reply. Perhaps if more people asked for that, they would provide us with some info). Here are links to some sludge scans concerning Testament:
http://www.metalsludge.tv/main/modules/subjects/pages/SludgeScansCareerTotals.htm Career total in the US:
1.123.801 as of
January 2004.
http://www.metalsludge.tv/main/modules/subjects/pages/SludgeScansDecember99.htm Live at the Fillmore in the US:
32.592 as of
December 1999.
http://www.blabbermouth.net/news.aspx?mode=Article&newsitemID=1871 Low:
112.36, The Gathering:
50.301 in the US as of
March 2002.
I've found 2 other sites that are publishing info on the sales of metal albums (basing on Soundscan):
http://www.metalinsider.net/category/metal-by-numbers,
http://www.metalsucks.net/category/neilstein-soundscam/, but they concentrate mainly on new releases. However, I suggest that you should contact them and ask for some info (I'm sure it's obtainable for them).
If you are to quote any information coming from Soundscan, please bear in mind that it pertains only to the US and concerns sales after May, 1991. I don't know where to seek info on the earlier sales (which is necessary in the case of The Legacy, The New Order, Practice What You Preach and Souls of Black, because probably a vast majority of their sales was before the introduction of SoundScan).
Deadly Embrace is a great source of any Testament-related knowledge, but he hasn't been posting here for a while. Perhaps Ms. A. (TL) could help you, as she also might have some info directly from the band (I don't know if that qualifies as a source for you). Anyway, if you happen to get any info on any of the albums, please share it here. Cheers.