There are shopkeepers who sell blueprints, check out wiki.
Sniper Rifle Blueprints are not purchasable from vendors as far as I have seen, I checked out the wiki, pretty sure they are only sold by players, found in lock boxes or gotten as a quest reward.
define "large amount of caps"
What I define as large others may not. I'm not Rockefeller rich but I've got quite a bit of caps put aside specifically for the Sniper Rifle BP's. Once someone has a set and talks to me we can deal from there.