In all honesty I don't think I have ever come across engagement gift thank-you here in the sunny UK. You can get bog standard thank you letters / cards thanking people for gifts but they usually have a wedding theme and / or picture.
Perhaps you could look into having some made and personalized by a stationers. If the budget looks thin then how about getting some card, glue, metallic shapes and glitter / paper flowers and metallic ink pens and have a bash at making some yourself?
I have had a look for you and all I can find is e-greetings and ironically enough a site that gives you 100 free bridal shower thank you with an order of wedding invites! Unfortunately with a subject like this the cards are usually very serious and a bit mundane hence the suggestion to make your own! It's hard to judge what you would like as obviously I don't know you personally but I get the feeling that you'd like something with a bit of life and sparkle. Not what you get with your run of the mill cards!
Remember you can always spruce up shop bought cards with some glitter and others bits. If you are way to busy with all of the other things that need to be taken care of just ask some friends or family to help you with it so that you can get some other things taken care of. But you will need to tell them what you would like on the card so they don't just make all these different kinds of cards and send them out to all of the people that were there. There is another idea for you, have all of the cards look different so that no one person will have the same kind of thank you card as someone else.
According to traditional etiquette, you and your husband-to-be should each have your own stationery on which to write pre-wedding thank you notes. In the most formal, traditional scenario, you ought to have fold-over note cards featuring your current monogram, and your husband-to-be should have panel-style correspondence cards with his own monogram.
If your style is somewhat more casual, you might consider thank you notes which simply feature your first names (Deb and David, for example). Note cards such as this may then be used both before and after the wedding for correspondence.
I am not an expert in etiquette but forgive me if your Mother's idea is generational. They threw the party for both of you; you are a "couple" in the 21st century. It all depends on how much your Mom will be teed off. But common sense tells me one card will do the job nicely. And I am a grandma, if you want to show some imagination, send a rhymed "thank you."
With some families they will leave the thank you cards for someone else to send out to all of the people that showed up to the wedding. Most of the time it will be done by either there mother or by someone that just wants to be a help as much as possible. But with a lot of people out there if you are going to send out thank you cards they would really like it if they had some kind of hand written note in the card so they know that the bride and the groom know that it was that person that gave them money or a gift. I know I would like to see something hand written in a thank you card from the bride and groom.