Dude.. seriously?
If you can get heroin, I would ASSUME that you could easily get other opiates, i.e. Oxycodone. Like one user said up above, if you're not willing to plug then don't bother. Personally, I don't think the risk is worth trying to IM heroin.
You claim that your intentions of wanting to IM heroin is solely so you won't be sick, so if that's the case... drop the H and go find other opiates! Heroin is an opiate, so if you can score a few Oxy's and you ingest them, or snort them, they will help you in not feeling sick from withdrawal. Don't expect a high if you've been abusing H for 3+ years, but like I said.. they'll help with withdrawal.
I can't help but draw a mental picture of what you look like in my head, and I can't help but say that you've reached a point where you need serious help. Not sure where you live, but if you put down the needle and go to a clinic, they'll help you! Opiate withdrawal isn't anything to joke with, so whoever you go see will make sure you're not sick anymore. But if you go to a clinic and live in Florida, just prepare to be placed in custody and forced into rehab under the Baker Act.