Available payment method for your order is calculated here :
src/src/app/code/core/Mage/Payment/Helper/Data.php::getStoreMethods()

And call
src/src/app/code/core/Mage/Payment/Model/Method/Abstract.php::isAvailable() && isApplicableToQuote()

Is there a function to get all available payment methods in magento ? YES

app/code/core/Mage/Payment/Model/Config.php

public function getActiveMethods($store=null)
    {
        $methods = array();
        $config = Mage::getStoreConfig('payment', $store);
        foreach ($config as $code => $methodConfig) {
            if (Mage::getStoreConfigFlag('payment/'.$code.'/active', $store)) {
                if (array_key_exists('model', $methodConfig)) {
                    $methodModel = Mage::getModel($methodConfig['model']);
                    if ($methodModel && $methodModel->getConfigData('active', $store)) {
                        $methods[$code] = $this->_getMethod($code, $methodConfig);
                    }
                }
            }
        }
        return $methods;
    }

To get payment method from an order, you can do this :

$order->getPayment()->getMethodInstance()->getCode();