Try Maisto. They have a lot of 1/12 scale cars and motorbikes. Used to dislay my Figma kiritsugu in a Harley big bike (smoking and holding a gun) and Archer in a red kawasaki if I remember correctly.
picture #985797 The bike is from Minichamps. Somewhat pricey however...
ex:ride has a nice set of pre-built motorcycles, mopeds and other designs. Since they were released by goodsmile ("freeing" brand) they are naturally scaled to figmas. They are old so sometimes a little hard to find but not impossible. I recently got a few ex:rides at retail or lower, brand new. Amiami has a few types on their site at retail price (which is pretty cheap compared to what they would cost if they did a re-run of them now, I'm sure). If you want to use a proxy, you may also want to try amazon.co.jp or yahoo auctions. And mandarake has them occassionally. With patience you can also get them on e-bay at a good price (but really, patience is required because most of the listings are ridiculous and never sell).
I would recommend Automaxx 1/12 scale. They are affordable and good looking (exept for the dashboard). I have a Kawasaki Ninja made by them in my shelf. The scale is the same as with item item #198695 . prices are around €20,- and they have models from different manufacturers.
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Aoshima have complete bikes: here
They also have models, with Fujimi and Hasegawa also: there
(Linking to HLJ just as a sample, you can find them on major retailers)
There's also the ex:ride series: encyclopedia #25367 (Max Factory, Y U NO MAKE BLACK TRIKE)