Have already tried 'standard' job posting sites.
Startup recruiting is really a full-contact sport that must be played to win by the Founder(s) of the Company. If you want to hire the best possible talent, you must be as focused on this task as you are a fundraising round (in other words, completely focused on it until it's DONE).
Great technical talent often has aversions to LinkedIn because of the sheer amount of inbound requests they get from recruiters. So although you shouldn't rely on it alone for making contact, it is still the best resource in finding potential talent. They give you such limited space to make your case in the introduction request so get the most compelling points about you, your company and why they should join you in the shortest possible amount of text and end with a call to action (Can I buy you lunch / coffee whatever).
Technical talent are far more likely to have multiple online presences so there's a better chance you can find their personal email address than recruiting other talent.
Be upfront as to your company's stage. If you have funding, make it clear that the position isn't subject to funding. If you are recruiting a cofounder, that should be made clear. In that case, focus especially on what makes you credible and attractive to partner with.
Happy to share more with you in a call about what I've learned in the process of hiring great talent.
Post a challenge with reward, although it's just a token prize but if the challenge is interesting (should be related to a problem you're facing now in the company so they can imagine what will the day-to-day work be), candidates will be attracted to it, not because of the prize but because they're intellectually stimulated.