Scale mates is a place to look for that sort of association between base kits and aftermarket accessories for that particular kit. Great Models webstore (before they were gobbled up by Sprue Brothers) had that sort of association as well, as long as they carried that particular AM item. Since you are working on an LAV, you may want to hit up some armor centric websites such as Perth, Armorama, etc. And ask in the armor forum here for "Trumpeter LAV Aftermarket?" specificly. There are some folks here who can probably help you out better than your very generic question header here.
As for me, I keep my AM requirements simple for stuff like stowage and such.
F is for FIRE, That burns down the whole town!
U is for URANIUM... BOMBS!
N is for NO SURVIVORS...
- Plankton
LSM