Tomato paste (puree) does not keep for long if stored in its opened tin unless it is plastic coated inside, if you keep it in a fridge it will last about a week. It is better to transfer it to a sterilised jar, it will then keep for a few weeks providing you use clean utensils to remove it. It is the wild yeast and bacteria that is all around which kicks it off.
There are two ways to get bulk orders. First I would suggest you approach your local Tesco, Sainsburys et al, as most of them will get you catering sizes or standard sizes in bulk, you normally have to buy a case/pack at a time. I buy my Besan flour from Tescos and sometimes other stuff in bulk, I just ring in an order with the code and I collect it from the store 2 or 3 days later. Another option is Bookers they are a national chain of wholsale stores, but you will need to get hold of a trade card/account.
Hope this helps
CP