If you are very creative or you have lots of time, you may want to consider making the invitations yourself. This allows you to be as original or imaginative as you wish to be and there are many excellent office supply stores who sell suitable paper, card and vellum paper. All you need to do is design your own template, get the relevant materials and away you go!
The same thing applies to order of service booklets, menus, evening invites, thank you cards etc. Just make sure you have plenty time to experiment before you have to set to work and produce the finished product. You also need to consider what size you want the invitations to turn out as it is very important that you have sourced suitable envelopes before you do the final draft. Another alternative might be to design your own cards, save it in a computer file and take it to an office supply store to have them printed professionally there.
If the idea of doing your own invites sounds too much like hard work, there are countless good stationary or wedding stores where you can pick out your invitations and order the required amount. There are invitations to suit all budgets out there, so try to shop around and find one that you both like and that suits your budget. Do remember to order a few extra invites in case you spoil any or in case of last minute decisions on who to invite!
Sending out your invitations 6 - 8 weeks before the wedding is an approximate guideline. Make the guest RSVP date approx 3 weeks before the wedding and even then it is inevitable that you will have to actually contact a number of them in order to find out if they intend to be there or not.