You lose two stops with the 2x converter so your tamaron with a 2x will
have a maximum aperture smaller than f/5.6 and the AF will not function.
If you want AF look at another avenue.

You will not be pleased with a 2x teleconvertor.  You will end up with a
56-400mm F/7.6-11.2.  Furthermore, you will not get better quality than just
cropping your picture by a factor of 2 and enlarging.  Chromatic aberations
will increase a lot.  It will be very hard to focus given that your
viewfinder is difficult to accurately manual focus already and with the
reduced aperture, your viewfinder will be quite a bit dimmer also, and the
apparent depth of field will be much larger, making it even harder to see
the plane of best focus.

Remember that you camera cannot autofocus with lenses faster than about f5.6
to 6.3 when they are wide open.

A Canon 75-300mm F/4-5.6 lens can be had for as little as $149 brand new
from Adorama.  It is quite light and small.  You could probably find a used
one for even less as many people sell it when they get something better.
http://www.adorama.com/Search-Results.tpl?page=searchresults&searchinfo=canon%20
75-300mm

add another $30 if you want the USM version (they are optically the same).
Don't waste $300 on the Canon 75-300, it really isn't worth it when it can
be had for half of that.

You could also consider the Sigma 70-300mm f/4-5.6 lenses at 124.95 and
199.95:
http://www.adorama.com/Search-Results.tpl?page=searchresults&searchinfo=sigma%20
70-300mm%20canon

The APO version is a little bit better particularily near the edges of the
frame.  Both of these focus down to 1:2 compared to 1:4 for the Canon, and
they go a bit wider.  The aperture also shrinks a bit slower as the focal
length increases.  Unfortunately they are both larger and heavier than the
Canon 75-300, but Sigma does include the lens hood which Canon charges extra
for.  Sigma also prints a distance scale and a magnification scale on the
lens whereas Canon unfortunately puts very little markings on their lenses
these days.

I have a canon digital rebel and a tamron 28-200 f3.8-5.6, I am looking at
buying a 2x teleconverter on eBay for under $100.

Are any brands better than others, are they bad news or should I just buy a
canon 75-300 for around $300 and not waste my money... ?

kk

[...]

Please post only simple plain text.  That's much easier to
read, as well.

You'll also need a 3rd party focussing screen and a _stable_
tripod. for you'll have a very marginal 54-400mm f/7.6-11
without any AF.  And a lot of light.

Canons will not fit most probably.

With any but the highest quality lenses, yes.  They enlarge
any faults, too!

Probably much better (though find out if the sharpness at 300mm
of the very lens you want to buy is enough for you before you buy).

-Wolfgang