I was initially unsure about 3D printable components, but my experience with thingiverse.com and treatstock.com was very positive. You can read my review of the entire process of ordering and receiving these brackets in my blog post, 3D Printable Components.

I've got 2 C64cs that, for whatever reason, don't have these brackets. In one of them, I've used a roll of insulation tape, (the kind meant for sealing outside door frames,) wedged underneath the keyboard and resting directly on the C64's mainboard. It's been like that for more years than I can remember. I guess because there was never a source for them when I was younger. I've got a second C64c where I'm using a piece of the Millennium Falcon's satellite dish to hold up the keyboard. Don't ask, it was my brother's model and I pilfered it long ago, before anyone cared about the revival of Star Wars.

I got these parts printed via TreatStock.com. They were printed right here in my home town, and were available less than 5 hours after I placed the order. I just drove over and picked them up after work the same day I ordered them. Fabulous.