You can probably only find a replacement by buying another SL1800.
There's a slight chance that a lot of manufacturers use the same design for basic OEM 'free gift' kinda tripods, you can try your luck and see if you can find a generic piece with similar measurements on ebay, but of course if it doesn't find you'll end up with two useless pieces. IMO, might be cheaper to write it off and get something decent.
If you wanna try, this one looks like it has a similar shape and claims to be "Spare Quick release plate Suitable for Velbon CX-444, CX-888, CX-460, CX-460mini, CX-470, CX-570, CX-690, DF-50 SONY VCT-D580RM, VCT-D680RM, VCT-R640". IF the SL1800 is a copy/OEM of these other tripods, there's a chance. Haha. See for yourself:
http://www.ebay.com/itm/Tripod-Quic...405?pt=LH_DefaultDomain_0&hash=item2ee1ab447d