I have found a source of lead shot with diameters up to 9.1mm (0.36").
 
What sizes do model makers want?

Cannon Balls
mmActual sizeWeight gNosPrice£
  9     9.1 - 0.36"        45       10     1.65
  7.5     7.6 - 0.30"        27       10     1.55
  6     6.0 - 0.24"        14       10     1.45
  5     5.1 - 0.20"          9       10     1.35
  4     4.1 - 0.16"        10e       24e     1.10
  3     3.1 - 0.12"        10e       60e     1.00
  2.5     2.4 - 0.09"        10e     120e     0.90
  2     2.0 - 0.08"        10e     200e     0.80
  Sizes 9 to 5 = Lead.   4 to 2 = Nickel plated Lead.
  Add 34p 1st Class postage.   Weights e are approximate.

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