You can register a .pl domain from the site 9cubehosting.com .Here they provide all international domains at low price.
Where to register a .PL domain name?
Really there are a lot of registrars, which provide domains in this zone. Look here (hereisdomain.com/pl) at prices set up by different registrars and may be you can choose something.
You don't have to use a Polish registrar, many big name international registrars offer .PL domains and often for a much lower price. Anyone can register without restriction. Try gandi.net for example, a known registrar that specializes in international domain names.