Don't worry too much about not being able to get to a sex shop, I've been in a couple and almost all the dildos they had were so ridiculously massive they must make most of their money off of gag gifts. Length doesn't really matter - you can put in as much or as little as you like, it's the width you have to worry about.

Make sure you get one with a flared base or with balls at the base - you don't want it to get lost inside of you! Scarily enough that can happen. For that reason, out of the two you mentioned I'd recommend the Trinity Vibes one.

This is the one I have, it's actually very thin. I like it because it's fun, rather than something that's challenging, and will mean I'm tight for when I actually do anal with a guy for realsies. As it's jelly it is a bit err... floppy, so putting it in can be a little awkward but it's fine after that. I put a condom on it so I don't have to worry about whether the material is healthy for me or not, as I've heard some mixed reports.

This one looks good too. I'd say don't go for something huge, go for something small and fun :)

Edit: LUBE! Lots of lube!

Edit 2: Looking at the other responses it looks very doubtful that it will come in packaging saying what it is, but you could always claim innocence and say it's a friend sending you a gag gift ;)