Attached are some websites that provide detail about random sampling from a surveying perspective. In general terms, you need to look at your sample size and the confidence level that you want to determine the appropriate size for the sample. Then, you need to use some type of random number generator to determine your starting point. For example, with 5000 products lets say that we need 100 products in our sample. We would then take the first 100 numbers generated and pull those products. Or, we could say that we need about 100/5000 which is about 1 in 50, so we could use the random generator to pick the starting number and pull every 50th product after that (and going back to the beginning).
Remember that even the numbers generated by random number generators on the computer are not 100% random.