Answered Where can I download the AliasID snippet, or is there an other way to retrieve an id from an alias?

If I'm understanding you, it would take a custom snippet, but you shouldn't need the ID. If you know the alias and the context you want, you can to this:

$alias='some alias';
$context = 'some context';

$resource = $modx->getObject('modResource', array('alias' => $alias, 'context_key' => $context));
if ($resource) {
    return $resource->getContent();
}

If the alias is unique (i.e., not duplicated in other contexts), you can do this instead:

$alias='some alias';

$resource = $modx->getObject('modResource', array('alias' => $alias));
if ($resource) {
    return $resource->getContent();
}

Note that you'll be getting the raw content field of the resource. Any MODX tags it contains won't be processed.

<?php
$alias='form-contact';
$context = $modx->context->key;

$resource = $modx->getObject('modResource', array('alias' => $alias, 'context_key' => $context));
if ($resource) {
    return $resource->getContent();
}