Serg, Start at ebay first...for everything.
Then if you are brave hit up some of the motorhead websites for parts. They have some of the higher end stuff on sale at prices RN and AB or any of the other sites can't touch.
For example. I got two Optima red tops for $90 ea from Summit last year. I put in the second battery kit (from Summit) for $100. So for the cost of one yellow top from RN or AB, I now have a :buttrock: battery system. I spent the extra money at partsexpress.com and got some zero gauge wire to wire it up.
Here is a pic of the batteries.
batts
I have pulled some crap out as I am doing some mods to the cooling system.