I dont really like using plant weights, I take it from you post that you need to do so probably because you are usung a larged sized gravel for your substrate and the plants wont stay in situ. If so buy some really small plantpots from your diy store and a sml bag of fine gravel from your Lfs. !/3 fill the pot with unwashed fine gravel and garden loam. then top off the pot with washed gravel. place in the aquarium and plant your plants into this (fine gravel will hold the stems easier whilst remain open to a certain degree unlike sand) then cover the pot and all with your normal gravel in your aquarium.
Regards
BigC