I'm planning on opening my hosting/designing company soon, and I'd like to offer domain name registration to my clients. The thing is I'm from Canada, so I would like to offer ".ca" domain names. Does anyone know any well known registrar companies that I would register with them, and would also work with WHMCS, since I am using that system for billing?
WHMCS doesn't support that many registrars, just go to the website of each one, see if they offer .ca domains for a good price and then choose the one you like the best.
You also can use for .ca domains an other registrar then for the other extensions, that can be done easily with WMCS.
I've had a look at enom and it seems quite expensive to register a reseller account with them. Is there a cheaper way to register with them?
Yes and No
If you want to become one of enom's resellers - i.e. a direct client known as an ETP, then no, you have to invest the money in your business.
If you simply want to be able to use the enom API and are happy to have an intermediary in the chain who you should use for 1st line support etc, than a reseller sub-account is what you need - which you can get from any reputable ETP, most of who offer them for free (subject to enoms min $100 "topup" system which is advance payments against domain regsitrations, not a *fee* for having the account)