Ironmind is the standard weapon of choice for most in grippers because they tend to be pretty well standardized for strengths - however, no two grippers will ever be exactly the same, so as many have found out, if you try someone's IM #1, for example, it may be slightly harder or easier than one you purchase.
Best bet is probably to go with the IronMind ones, starting with a Trainer and #1 gripper - most people get stopped by the #1 the first time they try it, even if they think that they have a strong grip. The #2 is something that very, very few have closed their first try, and the #3 is definitely not worth ordering from the start unless you have an aspiration to eventually be a grip champ. The pair of #3 grippers I own have never quite been closed without an assist from the other hand (1/6" from completely shut at best with a single hand when I've trained hard with them), and I'm not one to say that my grip is on the weaker side
Once you've had time to play around with what is the standard for grippers, I can definintely fill you in on where to go next for the levels up from there!