This answer is technically correct but perhaps not particularly useful - it is a long walk to reducing someone's saves. But if you can get them to Level 3 of Exhaustion, they will have Disadvantage on Saving Throws, your proximate goal.
So how, as a rogue, can you exhaust someone?
Chase them, or more likely, get them to chase you. Although only an optional rule in the DMG, if your DM is on board with it, a creature that Dashes enough times during a chase has to make a Con save - and failure imparts one level of exhaustion.
How to get them to chase you, especially enough times to fail some Con saves? As a rogue, that's fairly easy - use your superior Stealth and missile weapon Sneak Attack to snipe at them. When they see you - run away. If they refuse to give chase, Hide and snipe at them again. If they give chase, you can make sure they don't catch you with a Cunning Action Dash. If you are not doing enough damage with a single sneak attack, add some of your Poison to your arrows. Let them wear themselves out chasing you, and they will become an easier prey for your poisons.
As a rogue, you are literally built for this kind of Hide-and-Snipe action. Unfortunately, unless the rest of the party is similarly constructed, it will get very boring very fast for them. Elaborate guerilla tactics will not make you popular if the other PCs are sitting around waiting to bash.
KorvinStarmast suggests prompting them to chase you by taking something of utmost value to them, and letting them know you have it. As a rogue, you are also built for this - Stealth and Sleight of Hand.
Out of Battle
If you are more a 'slip it in their tea' kind of poisoner, consider loading up on soothsalts if they are available (and see V2Blast's comment below). One lozenge-sized dose of these imparts a single level of Exhaustion on a failed difficult Con save. A second dose could grant one or two more levels of exhaustion (depending on how your DM interprets 'cumulative'). Now, they are pricy at 150gp a pop, but if you are regularly tossing around things like alchemist's fire I'm guessing that's not a problem.
While both of these suggestions will achieve your proximate goal of lowering someone's Con saves, note that they do it by means of - requiring them to fail Con saves. Perhaps not the most satisfying answer for you, but they both work within the rules and within the parameters of your low-level but apparently wealthy rogue. At least you might use these tactics repeatedly to prepare them for the main event - failing their save against your much more important and expensive signature poison?