I have some Ideas for T-shirts and coffee mugs, but where can I get them made? I've got an account on redbubble, but the prices are steep. I was wondering if anyone else had some experience in this department?

Well, there's cafepress and topatoco. You could also check out some of your local printers if you're just interested in getting some swag made for conventions, since they can sometimes do small-ish quantities for very reasonable prices. Going local also gives you the advantage of having more control over the final product since you can request proofs and talk to the printer directly about every aspect of production.

Damn good advice Maskdt.

Bruno, you could also contact the suppliers of Promotional products. For example:
https://www.4imprint.com/mall/135/USA-Made-Promotional-Products
and http://www.usapromoitems.com/

Many of the things, like T-shirts, will be expensive to produce because their minimum orders are very large, but there are always other things that are quite cheap and definitely worth doing.

The artwork you have to provide for that sort of stuff tends to have to be pretty simplified: one colour art goes on a LOT more things, extra colours often cost more.
The amount of things that allow full colour digital prints are relatively small.