Rear Gel Filters...?

Hi all...

Just curious, and had a few questions about the Rear Gel filters, with lenses equipped to hold them. So here goes:

1) Where to buy them and what to 'look' for? B&H? 2Filter.com? 'Rear Gel Filters'?

2) Of what benefit are the Rear Gel Filters, over others?

3) What kinds of filters do they sell? I figure this will be answered, once I find a decent place to get them...

4) What types of rear gels do they make? and what are they made of?

Thanks in advance.

Does no one have any suggestions? Is it possible to buy a 3x3 gel filter, and cut it up? Would that be 'acceptable' or are there specialized filters, for the rear holders.

Let's say the lenses are:
EF 17-40L
EF 15mm Fish
300F4L IS? -- haven't checked to see if this has a filter holder...

Yes, you can (in fact are expected to) buy 3x3 gels and cut them up for the holders.

Thanks for the reply Jon. I figured the 3x3 were the way to go. Now, just to figure out if this method will be a little better than having to get a dedicated GND setup. Hmm...

Forget rear filters for grads. With a grad you need to be able to adjust where the front graduation falls. Same with polarizers; you need to be able to adjust them. Rear mounted filters are (barring special cases as in the jumbo fast super-teles) unadjustable.

Yeah, that was my thought. I have CPL's for most of the lenses I own, so that's not an issue. It's attempting to use them with GND's. Don't think that I would be able to use a CPL attached to the lens, alongside a GND. I have seen some of the setups here, and can't tell whether a slim CPL on the 17-40 would work at all, with a holder and some GND's. Though, by the looks of it, I won't be able to use it.