<edit> I see you are in Spain - Do you have commercial patterns over there? If not you might try ordering one from here, they don't weigh much and lots of online stores have international shipping. You may need to adjust them, so first make a mockup with cheap fabric and adjust it. If they aren't small enough, cut out the smallest size and shrink it on a copy machine.
Speaking from experience - by the time you buy the fabric, needles, thread, pattern, etc. and figure it all out, you might as well buy the PJ's you like. Sewing with knits isn't exactly difficult, but it is different than sewing with cotton and most people prefer to use a serger. I haven't really attempted knits even though I sew every day because I don't want to buy a serger or special needles. For the $18-20 I can buy a nice shirt for (for myself, or for the pups!) it's just not worth the trial and error. You might splurge on the PJ's, and try to sew a harness instead, which are made out of cotton and a bit easier to work with
Pet Accessories : Simplicity, Patterns for Sewing Projects