I think Forged in Battle are the cheapest and best quality out there at the moment - for Early War Poles. They do not however have everything you need. Battlefront still has very good vehicles (7TP and Tankettes) and the artillery is actually quite nice if you can look past the very thick metal bases on all infantry which I tend to either cut away or trim heavily. So FiB infantry and cavalry, Battlefront vehicles and guns is my main choice. Of course Peter Pig has some nice and cheap vehicles as well which the Poles get to use so I've bought stuff from there as well.
FiB are slowly expanding their range, and have released stuff that really rivals other manufacturers, the 10th motorized from FiB are the best looking on the market imo, and their imminent release of the dismounted cavalry is something I really look forward to as those models are gorgeous.
But playing with Poles in 15mm at the moment forces you to buy models from various sources.
My army comes from the following places:
90% of all infantry = Forged in Battle
1 HMG platoon = Battlefront
AT guns and artillery = Battlefront
Tankettes and 7TP tanks ) = Battlefront
Armoured train = Battlefront
FT-17 and R35 tanks = Peter Pig
Some infantry and 1 cavalry platoon = True North/Old Glory
Karas PZL.27 light bomber = Armaments in Minature
What really kills me is that no one makes good and detailed looking cavalry Taczankas, Fiat 508 Taczankas, Vickers E type A and B tanks and a few other minor things like AA machineguns, crewed Bofors guns. There are manufacturers out there who make some of the above mentioned models but they are just simply not good enough imo...
The FiB infantry is cheap, but you really have to add a bit to the cost when you take separate bases into account. I still use Battlefront original bases since that is what I started with for this army. Though I have recommended
http://warbases.co.uk/ for cheap bases (and it's the source from where I buy bases for my By Fire & Sword project).