I am currently obsessed with multiwear/convertible/infinity clothes ever since I first owned an Infinitude
Now, if you have a trusted modista, and you're not model lean... i'd highly encourage you to just have one made (multiwear dress), just so you'd have straps that are the right width and length for your particular body type. Here is a really helpful tutorial http://sewlikemymom.com/little-red-infinity-dress-tutorial/
There's plenty of 'spandex' fabrics in Divi (P120-140/yard) or Fabric Warehouse (starts at P180/yard) you can buy... of varying thickness and bagsak... but the good thing about this fabric nga is it stretches, doesn't fray and doesn't really need to be ironed. Fabrics are cheaper still at taytay
Fabrics you might want to use include crepe, matte jersey, cotton knit, jersey knit (but usually, sa Divi, they all fall under SPANDEX, hehe)
oh and one something to consider in such multiwear tops... either you go braless or get special bras
there are many basic patterns ng multiwear tops, some start with a bandeau where you can wear a regular strapless bra... but some others talaga, the straps will make the top, so hindi pwedeng lagyan ng padding or support...