Your best bet is a YLOD PS3 off of ebay and ask the seller when he or she last updated. You pick up a YLOD PS3 for about £30 and once you fix it, if it's not on Firmware 3.41 then you can go and exchange it and HEY you've made a profit which you can then use the money to buy a brand new one or repeat the process. I just done this method and got a PS3 with YLOD on Firmware 3.41 with a faulty Blu-Ray drive, I now have two PS3's on Firmware 3.41
EDIT: You can also go to an exchange store and ask them what's the longest PS3 they have had in stock and it will probably be on Firmware 3.41 or below, or, go into any store and ask them what's the longest PS3 they have had in their stock, as long as you are specific and you are definitely going to buy it then I'm sure they won't mind helping you.