I hope you understand that buying and installing iWeb on your new iMac will not bring back any of your iWeb files if you have not backed them up? If you don't have any of your domain.sites files, then they are now lost with your old Mac and you can't get them back and if you do install iWeb, then you are looking at starting to re-build your site from scratch?
Yes, you can still get iWeb, but as Apple no longer sells it you will have to buy it from Amazon and you will need to buy either the iLife 09 or iLife 11 boxed set which contains iWeb 09 and then install it on your new Mac. You can just install iWeb if you want to without all the rest.
A very good alternative that I have found is somethign called WebAcappella. It comes from a French comapny and they have not long brought out WebAcappella 4 and it is the best alternative that I have found yet to iWeb. It has all the things built in that everyone wished that iWeb had. Has built in drop down menus, contact forms and Google maps and the ability to add your own html into a snippet/widget, just as iWeb did. You can use templates or start with a blank page and you can also create page templates that allow you to put headers and navigation menus on all your other pages with just one click without the need for copying and pasting in iWeb that could be very time consuming.
It is not the cheapest, but if you go to the website at http://www.webacappella.fr, then you can download it and try it out and you can create up to 8 pages with the free edition. If you then like it enough to buy it, then your just purchase a licence key and it is about 70 Euros.
I have looked at programmes like RapidWeaver, Sandvox and Freeway Express in the past and don't really like them. I have been using Flux 4, but unless you know a little about coding, then this can be quite a steep learning curve. Freeway Express has a new version 6 however, so that might be worth another look.
I would urge you to look at WebAceppalla though, as it does seem to be a good alternative and you are not tied into a hosting package either - you can publish where you want, unlike the new product from Rage that will be tied to Rage webhosting when it finally appears.