After looking for months i finally found the Sentinal XS and i found it while in a mission and was able to keep it after leaving the mission.
I've had the Hardtop Convertible one for a couple weeks now and then i began looking for the XS but they aren't together because i found the XS with my second char.
This is when i wish our char shared cars too but oh well i'm happy that i finally found it.
Here it is, I haven't modded it yet the only downside is i'm only a lvl 13 right now so i don't have any good mods unlocked yet.
if you want to easily find one for your other character, go to the LSC in the centre of town... stand at the interesection right in the middle, get your autoshotty ready..
then press pause, go to the ps/xbl store, then back out... you will guanrentee a sentinel dude to spwan and drive past you, you can tell which direction hes coming from (usually behind you) from hsi loud music, take aim, autoshotty his brains out, take car to LSc and track...
Good for you, I'm the opposite, been trying to find the sentinel convertible for months but no luck..
if you want the convertable do the above, then drive it between the golf course and weazel plaza, youll probably find one within about 10 mins... easiest way to find them is do it at high noon on a sunny day, its much easier to spot cos the NPC's like to take the top down...