Most American factory loads are emasculated in defference to idiots who don't know the difference between rifles designed for 8x57J and 8x57JS rounds (8x57J was limited to a much lower pressure). The older 8x57J used a .318" 220 gr round nose bullet at a MV of around 2200 fps. The 8x57JS uses a .323" 154 gr spitzer bullet (military load from WWII) at almost 2900 fps. Current Winchester, Federal and Remington factory loads all drive 170 gr spitzers at 2350 fps. Assuming your rifle was simply a 30-06 design altered to to take the bigger bullet and slightly different cartridge shape, there's no reason you can't equal or exceed the old WWII military ballistics with hand loads.
There's lots of surplus ammo out there, but with a new (i.e. not surplus) rifle, I would be very careful to stay away from corrosively primed ammo (pretty much all Turkish, and most Yugo). If it's mil-surp ammo, don't take the sellers word for it being non-corrosive. I've seen some ads on gunbroker.com for 8mm Mauser that the seller said was non-corrosive, but almost certainly was corrosive.
Some people here on THR think the corrosive is fine to shoot, and I certainly won't stop them, but my personal experience with the Turkish ammo is that it's so dirty, that corrosive or not, it's such a pain to clean that I've quite shooting my 8mm until I can get some clean, non-corrosive ammo. I've still got 300-400 rounds of the corrosive stuff that is currently being used as counterweight on my TV to keep it ballanced in a bookcase until I get the entertainment center built (the shelf it currently on needs to be about 1/2" deeper).
For factory loads, look to European manufactuerers for hotter loads. I know S&B loads 198gr bullets in 8x57JS (they also load for 8x57J which incidentally is a rimmed case so make sure you get the right one), but I have no idea how hot those loads are. I'm sure Norma loads for 8x57JS, be prepared to pay out the a$$ if you go with them. I think there's a fairly new load here (Hot Shot) that's made in Bosnia, but again I don't know how hot they are (they are very cheap though - around $8/20 for FMJ).
Your best bet for hot loads (relative to American factory loads at least) is to roll your own. When I don't have quite so much else going on I intend to start handloading myself, but in the meantime I know a guy (FFL) that does custom loads and I'm probably going to get him to work up a load for me. I think for the first order he's going to charge me about $74/100, and thereafter (if I bring the brass back) it'll drop to around $52/100. Still kind of 'spensive, but also a bunch cheaper than equivalent factory loads.
Good Luck.
Edit: to answer your question about online places to buy ammo, try these:
http://www.cheaperthandirt.com/
http://gunbroker.com/auction/Browse.asp?Cat=3017 (do a search on 8mm or 8x57)
http://www.auctionarms.com/search/auctions(1).aspx?ab=14 (do a search on 8mm or 8x57)
I'm sure there's lots of other places, but those were the first to spring to mind.