National Australia Bank is scouting for more banks to buy in the United States in order to improve the reach of Great Western Bank in select Midwest states.Doug Bass, regional president in Iowa for Great Western, told the Des Moines Register that the bank wants to have the top two or three market share spot in every state where it has a branch. Through Great Western, NAB has branches in Arizona, Colorado, Iowa, Missouri, Nebraska and South Dakota.Under the bank's ownership, NAB has lifted assets to US$8.5 million, or six times the asset base five years ago.NAB has plenty of work to do to realise this aspiration, with divergent concentration ratios in each state market where the bank operates.-- In Nebraska, Great Western's market share of deposits is 5.2 per cent, according to Federal Deposit Insurance Corp data. The top three banks in the state have a combined market share of 29 per cent.-- In Iowa, its market share is 2.2 per cent, and this takes into account the recently announced takeover of First Federal Savings Bank of Iowa. The top three banks have a combined market share of 20 per cent.-- In South Dakota, its market share is 1.6 one per cent, which is sufficient to count as the second largest bank by deposits in the thinly populated state. Wells Fargo accounts for four out of every five dollars in deposits in the state where Great Western has its headquarters.-- In Colorado, its market share is less than one per cent. The top three banks have a combined market share of 45 per cent.-- In Arizona, its market share is one quarter of one per cent. The top three banks have a combined market share of 70 per cent.-- In Missouri, its market share is one seventh of one per cent. The top three banks have a combined market share of 29 per cent.To reach the target of becoming the number three bank in each state in which it has branches (and based on the present branch footprint), NAB will need to buy banks with US$38 billion in assets, a four-fold increase on its present size.To become the number two bank in each state, NAB will need to buy banks with US$45 billion in assets, a five-fold increase.