I'm guessing you've got someone fielding snipers in high numbers, because they really shouldn't be doing that if there's just one around (caveat: The Sanctus may do it to Psykers). This suggests that you've been running lists fairly character-heavy and may want to change things up occasionally to punish that (over)investment in character killers. All of this is going to be general advice since you've not given many specifics, so here goes:
The good news, the ability to target characters is a good part of the assassin's cost so they're usually easy enough to deal with if you can get to them.
Not a Chaos player myself, but from the top of my head you don't have ready access to a bodyguard unit (one that happily takes mortal wounds to prevent a wound on a character) so that's out. Counter-sniping you'd probably need to ally something else in for as well.
So, getting to them. Some of the easier solutions left:
1. Transports. Both snipers are mere S4 weapons that only get their 2+ to wound against infantry. Hiding out in a bunker on wheels will make them wish they had spent the points on anti-tank weaponry instead. Rhinos are cheap.
2. Deep Strike. You have an abundance of choice here. Keep something sufficiently killy in Reserve to go hunting assassins. I'd suggest some Raptors with plasma, but Chaos has OPTIONS here.
3. Speed. Same idea.
Further mitigation strategies:
1. Snipers are far less of a danger to things that don't have the INFANTRY keyword, as they effectively only get 1 shot at S4 plus some occasional mortal wounds (note Vindicare's Deadshot doesn't trigger if no damage is done). Thus, fill out your HQ ranks with T5+ Characters that are not-infantry and that have enough Wounds that they can take it. Daemon Princes and Greater Daemons make decent candidates (all monster rather than infantry) and the new Lord Discordant is a good one as well (vehicle). Hell, have your Warlord be a Renegade Knight and wish your enemy's assassin's good luck with getting the warlord kill.
2. FNP saves help a lot as well.
3. Cover. Buffing characters don't need to be out in the open, and the sniper can't shoot you if he can't see you. At the very least you should be able to force him to take an awkward forward position. Plus, AP-3 means a Chaos Marine in cover should still at least get a 5+ armour save.
EDIT: For advice more aimed at your specific situation, please add some army lists or other details so we know what your specific situation IS.