First of all, remember that just because you’re having 100 guests in total, you won’t need 100 invitations. Couples and families will receive one invitation between them, so you might only be looking at buying around 60 invitations in total, and 50 place cards for your day guests.
The Confetti shop has some very reasonably priced invitations so it’s definitely worth having a look, particularly at those where you write your own inserts instead of paying to have them printed. You could also speak to members of the bridal party (your parents, bridesmaids etc), explain your predicament, and ask if they don’t mind not receiving an actual invitation.
When it comes to placecards, I think you’ll have to get creative! Placecards can easily be produced on a computer. Alternatively, how about doing something like writing names on pebbles, or straight on to your favours.
I hope this helps!