While some will wince, I go a cheaper filter route with a gelatin filter holder for the Cokin line. So far I just use it with the A size holders. Larger holders, if you can fine the gelatine frame should work just the same.

Rosco (USA vendor) sells a very broad line of gels for theatrical (usually a two or three digit designation) and cinematography uses (usually a four digit designation).
The normal smallest sheets are about $6 for a piece about 24x30". Lee (UK vendor) also sells the same sort of stuff, with a differing naming scheme. There are also other vendors - GAM is one that comes to mind, and having some unique products.

The real deal though is a thing called a designer swatch book. Rosco's has over 100 filters in about a 3"x5" size, with a card behind each one showing the transmision curve.
I think I got mine for $20. B&H sells them, as well as individual sheets. I clip out the pieces to the reuired dimensions and mount them in the gelatin holder clips I have when I want to take them to the field.

I know they are not the finest optical quality. They are good enough for the bulk of my projects, where I cannot locate a 3x3 Wratteen filter from the accrued stash of them to suit the desired effect.