Here is a way that I think will work.

Let's say the item identifiers are in cells A1:Am and their associated weights are in cells B1:Bm.

1. Put the following formula in cell C1: =B1*rand()

2. Copy cell C1 and paste into cells C2:Cm.

3. Copy cells C1:Cm and Paste Special-Values into C1:Cm, over the same cells. This fixes the random numbers in these cells so they now will not change.

4. Sort the data on column C and select the top n items.