I don't known if this is US only, though it think it's only the OGP Game Cards, but the best and safest way to earning astros is by going to a Rite Aid store, somewhere near you, and buy an Ultimate Game Card for $20 which is worth 400 astros.
Be warned though, it seems OGP has the monthly astros purchase system back again meaning you can only buy $100 worth of astros per month (which is 2000 astros) which sucks in my opinion.
You can also use Credit Cards or PayPal or some other online paying method.
I hope this helps.
(Currently I have two Ultimate Game Cards waiting to be use to earn more astros. OGP you suck with this monthly payment system! Why the hell do you even have it!?)
"Invoke... VIRUS SOUL!!!" ~ From my VirusChris manga project