In your For/Next loop you are iteratively selecting (or trying to select) each entry in the Supplier drop-down, is this your intention or do you want to select either a specific Supplier or a random Supplier?
Something like this should work for a random selection:
You may have to spy on the Supplier drop-down and change the name value if this is incorrect
iSupplierCount = .WebList("name:=P1914155963_PG0_PC2_pv_source").GetROProperty("items count")
iRandNum = RandomNumber(0, iSupplierCount - 1)
.WebList("name:=P1914155963_PG0_PC2_pv_source").Select "#" & iRandNum