We use an oil column heater with a thermomstat to keep our DS's room between about 19C and 21C. They're safe as long as they are electrically sound and you position them out in the open away from any potential ignition sources (eg. curtains). And of course, turn it off if you're not home.
Once your LO starts crawling, it's also important to make sure they can't access the heater, in case of burns or pulling it over onto themself.
Target had some for sale last year.
Oh, and be prepared for a higher power bill!