I've seen a lot of smaller outstaffing companies like moonlightwork.com offering remote employees and it's a good way to keep your costs low. The lights will go out quickly if you get developers at $50-$100 an hour and upwork is hit or miss and you can waste a lot of time. Toptal seems good but not cheap. Check out intellabridge.com they have good mid-level remote developers you can hire for $18-$20 an hour and they specialize in designing and building MVPs.