depends how much power you already have and how much you plan to add and also if you still want to be able to wheelie easily on demand.
with a mostly stock blaster a +4 will pretty much keep you grounded at all times and make it hard to wheelie over obstacles. unless your only plan to hill climb with it i would not go over +2 on a mostly stock machine. honestly i'd stick with a stock swinger if your not doing any power mods
with an averagely moded blaster (pipe, reeds, filter) a +4 will still keep you grounded most of the time but with a little coaxing you should be able to wheely over SMALL obstacles like small puddles and small branches that have fell onto the path but a +2 would be the best of both worlds, letting you wheelie over bigger obstacles and helping to keep the front down a little bit on hill climbs
with a hevily moded blaster (reeds, pipe, filter, port work, big bore, stroker crank) a +4 will be a world of fun and wheelie on command over whatever you need to and still help keep your front down when hill climbing.
i say this because the +4 was one of the first things i bought for my blaster before i started adding alot of power mods, at first it was great because i wasnt afraid it was going to flip backwards on me all the time but soon after i started to like it less and less as i learned how to ride and realized it was actually slowing me down because i couldnt wheelie over puddles and stuff like everyone else. after adding some power mods i started to like it more but still regretted buying it a bit and wished i would have got the +2 instead. now that im hevily modded im loving every minute of the +4. i've actually been thinking about putting a stock swinger back on for a couple rides to see if i can actually keep it down with all that power now that my riding skills have improved.
if you do go with +4 on a mostly stock machine i'd atleast change your gearing more torwards accelleration incase you really need to wheelie over something.
i once hit a foot deep puddle at the top of 4th gear that threw me half way up over the bars, and hung my head right infront of the bumper headed for a tree because my breaks werent good enough to get me stopped in time and i was mostly stock wtih the +4 so i didnt have the power to stand it up and hydroplane over it. if it would have went 2 more feet my head would have been crushed between my front bumper and a tree....
as for where and who. i bought mine from swingarmshop on ebay for about $200 and am very happy with it. they make it any length you want from +1 to +6 or +8 for the same price