As a former indie iOS dev, ( ie. a tool for Apple Computer Inc. ), my honest feedback is- if I was ever making enough on the App Store to afford a $15 monthly subscription, I wouldn't need the help; for the most part indie iOS developers don't make jack so our #1 consideration for any product is going to be "Is it free...?"
Of course if you already figured that out; but saw that people were thinking "Why the hell am I not making any money on the App Store, maybe I need; like; SEO?" and are likely to give it a try- even if they don't like it, even if it doesn't work, you still got $15 out of that customer which is 10x what they're making on each customer who downloads their $1.99 app; so- good on you.
I realize it is a different target market but often SEO/search marketing/email/retargeting solutions can cost thousands of dollars upfront, without any guarantee of a return.
- still at the idea stage, I began talking to people online on iOS dev forums - iphonedevsdk forum, and reddit.com/r/ios /r/iphone, etc
- I asked my friends who I knew were devs. I live in Poland, and my target audience is mostly US, so their feedback was slightly limited, but still valuable, because I could talk to them in real life
- as soon as possible I went to San Francisco, and went to any meetup I could find through my contacts, on Meetup.com, and Startup Digest
- I try to follow news as closely as possible (e.g. there is iOS dev weekly newsletter), and look for opportunities to engage in the communication. Even without mentioning the name of my project (which is AppCodes.com -- a shameless plug here :D )
- Sometimes I write to bloggers and people who write about my subjects about something I work on, to honestly gather their feedback, and of course to ask if they want to know more about my project. It's always personal.
After a few years of doing various projects, I noticed that it almost always takes around 6 months for the word to go out that I do things, and people start coming back to me by themselves ("are you still doing X?"). With time, finding connections is easier, but not faster - it always takes exactly 6 months :)