Air shocks again...geeze. To me the best answer for a Sporty is a good set of Progressive shocks. The shocks are available in all different lengths, you can raise or lower the bike with different length shocks.

If you are REALLY dead set on air shocks, look around in the Touring section here. A lot of those folks are looking to remove the air shocks for spring shocks. Personally, I like the air shocks on my Ultra Classic, but I wouldn't want them on my Sporty.

 

Ignorance can be bliss! They are an awful bolt-on that reduces the effective length of a shock. Steer clear! If you are going to buy replacement shocks get them the correct length.

I have a SuperLow, which is similar to your Custom. IMHO the rear suspension is too low and will benefit from being lifted with 1" longer shocks. They will also give longer travel and should improve ride quality. For top drawer shocks call Howard of HDF sponsor Motorcycle Metal, as he will custom-build you a set. He is an Ohlins dealer, also has other brands.

You may also find the forks sag too much, so you have little suspension travel left. I replaced my springs with RaceTech, who supply different rate springs to match your weight, and set the sag correct which raised the front around an inch as well. An indy can do that for you if you are not confident of tackling the job yourself.