I took my 870 apart today but managed to break what I believe is the magazine spring retainer. It's the part inside the end of the magazine and holds the spring in place. Once the metal magazine cap is unscrewed you can see it.

However, when looking up replacement parts online I noticed none of the replacement magazine spring retainers look like the one that I broke. The one I broke is about 1 1/4" in length, plastic, and recessed on the inside to neatly accept the spring.

Anyone know why?

You have the new style magazine retention system.

The old system was a split steel "spring-cup" that was simply pressed into the magazine tube.

The new style is a long plastic tube that interlocks with two studs that are pressed into the magazine near the front end.

Here's a link to a Remington owner's manual.  It shows both the old and new style retention systems:
www.remington.com/support/parts/parts_schematics.asp

Manuals are on the right.

Thanks

Do you know of any sites online where I can buy the new style spring retainer? All the sites I'm pulling up sell old style spring retainers.

I don't know that they are neccessary.  It's been a while since I used a 5 round tube, but I don't remember anybody not using it having a problem after the retainer was lost/broken/just not installed.  All it does is make it easier to screw the mag cap back on so that you don't have the mag spring coming out when you want to remove the barrel for cleaning.  Without the retainer, the mag cap pretty much does the same thing.

The new style retainers are easier to remove/replace with a screwdriver and are safer in the respect that you don't need to hold your thumb over one edge while prying up on and around the three split ends of the old steel cupped ones. They often flew off launching the spring and the retainer across the room with force and I would imagine, a liability hazard if it hit someone in the eye.

I have a couple of drywall dents in my gun fixing room to attest to that from the old style
mag spring retainer getting away..

"Do you know of any sites online where I can buy the new style spring retainer?"

Direct from Remington:
www.remington.com/

Brownell's (Click on the Gun Parts Tab)
www.brownells.com/