The cheapest bullets will be ones that you cast yourself, if you want to go that route. I don't cast bullets myself, but I imagine you could probably find all the lead you need at a tire shop by using wheel weights. I don't know how well that would work though.
The 8mm bullets I use are the Remington core lokt. You can find them here.
"Cheapest" can be misleading. Bullets are heavy suckers and as the price of shipping has gone through the roof, I've found that purchasing them locally is the cheapest way to do it, even with sales tax thrown in. Unless you are going to buy a whole mess of bullets and brass at one whack, it might be better to head for you local supply shop, even if they have to special order it. That being said, take a look at Grafs. They include shipping costs in their price, so it's not a surprise at checkout.
+1 for graf's & casting. You can make brass from 30-06/270 just by trimming them to size & running them into the 8mm FL sizer (if you can get once fired brass at the range for free).With the current costs of powder/primers/bullets, buying surplus (if you can find it )is still cheaper.Even if you pull them down for the components only.per 100, new bullets will be at least $15, primers $2,& powder about $10-$12($27/100), so $4 a box($20/100) for surplus doesn't sound so bad.
I agree that 8mm brass can be easily made from 30-06 brass. A word of caution on using wheel weights though for lead bullets. All lead contains some amounts of arsenic so be sure to work in a well ventilated area. Also, watch out for the shiny wheel weights... it likely has too much zinc to properly cast. THe beauty of making your own ammo is that you no longer need to be concerned with corrosive ammo