There are several dating and friendship websites targeted towards people on the autism spectrum, including http://www.autisticdating.net/ , http://www.aspie-singles.com , http://www.aspieology.com , https://www.spectrumsingles.com/ , and http://www.autisticpersonals.com/ .
As others have mentioned, one can also find people with autism spectrum disorders on websites catering to people with disabilities, such as http://www.disabledpassions.com/ , http://www.disabledunited.com/ , http://dating4disabled.com/ and http://www.whispers4u.com/ .
There are (or were) Asperger's and autism dating groups on Facebook. See this old thread
http://www.wrongplanet.net/postp3358931.html .
Finding for a romantic partner who is also on the autism spectrum, however, tends to be difficult because people on the autism spectrum make up a small minority of the general population (roughly 1%) and there appears to be a gender imbalance (at least when it comes to diagnosis). If you then narrow your search by demographic characteristics (e.g. acceptable age range, religion, etc.) and geographical location, there may be no one left.
Finally, keep in mind that many people find online dating to be difficult, and so you should not expect any miracles.
(Disclaimer. I am not affiliated in any manner with any of the websites mentioned in this post, and hence I can not vouch for them. Caveat emptor.)