I see your dilemma. If you get a filter and better heater for your 10gal tank now (which it sounds like you could really use), you'll just have to upgrade again later when you get your bigger tank. If you buy a filter and heater that will be suitable for your bigger tank in the future, it will probably too strong for your little tank right now.
As far as the heater goes, you could still go ahead and get a decent one for the 10gal tank that's maybe one size higher like a Visitherm Stealth 100W or a 150W. Then when you get your bigger tank you could still get another heater and use both together. Sometimes it's better to have two lower wattage heaters rather than one big one in case one should fail, the resulting temperature change would not be as quick or drastic.
You could also do the same thing for the filter, perhaps buy one now that you could also use together with another bigger filter when you get your new tank. It's also nice to have spare equipment should your main filter or heater break, then you have a temporary backup until you can get a suitable replacement. Or if you ever need to setup a "hospital" or quarantine tank should one of your turtles get sick.
There are some great places online where you can get everything you need but I'm not sure how much more they charge to ship to Mexico. I wouldn't think it would be that much since delivery trucks go back and forth across the border fairly easily.
http://www.bigalsonline.com has really good prices on heaters and filters.
http://www.reptilesupply.com is a good place to find UVB bulbs.