The least expensive way is to luck into one at a yard sale. If you know any pipe smokers, you might get one that way. Also try a smoke shop that caters to pipe smokers. If they don't have an empty can you might consider buying a cheap brand in a can (since it's the can you want anyway) & give or toss the tobacco away. This might be cheaper than buying a can thru the internet plus shipping.

An alternative can would be a Sucrets can. They are making one called 'Sucrets Complete' which is a can somewhat larger than the regular Sucrets can (or Altoids.) Check the medicine aisles.

If you can find any of the old Bandaids cans (Again, try yard sales) some of them are roughly the same size as tobacco can, & would make an appropiate FAK container.

If you come across any of the 'Fossil Watch' tins those could make a nice PSK container. Some are hinged & some aren't.

Good luck seaching,
Tim